diff --git a/.dockerignore b/.dockerignore
index 09c8786..5de4c11 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -23,6 +23,8 @@
 !scraper/**
 !peering/
 !peering/**
+!perf/
+!perf/**
 !tools/
 !tools/**
 scraper/.cache/**
diff --git a/AGENTS.md b/AGENTS.md
index a138ee7..2bc9722 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -12,6 +12,7 @@ Guide for contributors and coding agents. Users who only want to run it: [docs/d
 | `scraper/` | Download queue daemon (`service.js`, `daemon.js`), source adapters and parsers, the publisher that writes the library, metadata lookups |
 | `peering/` | Optional server-to-server sharing (gateway, client, adapter) |
 | `runtime/` | In-container supervisor and configuration |
+| `perf/` | Switchable performance trace and chapter read-ahead ([docs/performance.md](docs/performance.md)) |
 | `discovery/`, `greatest/` | Discovery Scout and the Greatest of All Time dashboard |
 | `tools/` | Demo library, health checks, maintenance tools (dry run by default), UI and Compose test drivers |
 | `deploy/` | `circus.sh` operations helper, HTTPS with Caddy |
diff --git a/Dockerfile b/Dockerfile
index 33a0451..656cfc4 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,6 +11,7 @@ COPY tools ./tools
 COPY scraper ./scraper
 COPY peering ./peering
 COPY runtime ./runtime
+COPY perf ./perf
 RUN mkdir -p /library /data && chown node:node /data /library
 ENV PORT=8080 LIBRARY=/library DATA=/data TZ=UTC
 EXPOSE 8080
diff --git a/README.md b/README.md
index 2b5273e..a1d9700 100644
--- a/README.md
+++ b/README.md
@@ -63,6 +63,8 @@ you put in `.env` are listed in [docs/deployment.md](docs/deployment.md#4-config
 | `SIGNUP`        | `closed`    | `open` lets anyone create an account. Otherwise the admin adds users under Settings |
 | `SCAN_INTERVAL` | `10`        | Minutes between shallow library change checks (`0` disables this timer) |
 | `FULL_SCAN_INTERVAL` | `1440` | Minutes between full library reconciliations (`0` disables this timer) |
+| `READ_AHEAD` | `1` | Read an opened chapter's pages (then the next chapter's) into the OS cache ahead of the browser; `0` = off ([docs/performance.md](docs/performance.md)) |
+| `PERF_TRACE` | `0` | `1` = performance trace from the start; it can also be switched at runtime with the file `DATA/perf/trace.on` ([docs/performance.md](docs/performance.md)) |
 | `SCAN_CONCURRENCY` | `8` | Chapter folders and images probed at once by an uncached scan (1–64); higher helps on network storage |
 | `COOKIE_SECURE` | –           | `1` marks the login cookie `Secure` (use it when you serve over HTTPS) |
 | `SCRAPER_URL`   | `http://127.0.0.1:8091` | Private scraper daemon URL |
diff --git a/docs/architecture.md b/docs/architecture.md
index 7297d1e..3676450 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -210,6 +210,7 @@ All requests and responses are JSON. POSTs require `Content-Type: application/js
 | `POST /api/moments` `{m, c, v, pages, layout, region, cat, note}` | user | Save a moment: `pages` are 1–4 distinct indexes in displayed order, `layout` `row` (default) or `column`, `cat` one of `twist hot awesome cute funny feels` (required), `region` fractions of the pages put together, or null |
 | `POST /api/moments/update` `{id, cat?, region?, note?}` · `POST /api/moments/delete` `{id}` | user | Edit or delete one of the caller's own moments (another user's id is `404`) |
 | `GET /api/notifications` · `POST /api/notifications/read` `{id \| all}` | user | Notification list and read state |
+| `POST /api/perf` `{page, events: […]}` | user | Browser timings for the performance trace; ignored with `{on: false}` while the trace is off. `/api/chapter` returns `perf: true` while it is on ([performance.md](performance.md)) |
 | `GET /api/admin` | admin | Users, last scan result, library path |
 | `POST /api/admin/rescan` `{full}` | admin | Scan now, and wait for the result |
 | `POST /api/admin/user` `{name, password, admin}` | admin | Create a user or reset a password |
@@ -400,6 +401,7 @@ docker compose up -d --build
 | User data | `db.json` is rewritten as a whole, at most about twice per second while people read. Comfortable up to a few MB (a household or small group, thousands of chapters read) |
 | Concurrency | One process, one writer. No clustering or horizontal scaling by design |
 | Images | Served as stored. No resizing or thumbnails, so large covers make the library page heavy |
+| Slow storage | Opening a chapter reads its pages (then the next chapter's) into the OS cache, two at a time (`READ_AHEAD`). Where the time goes: [performance.md](performance.md) |
 
 ## 10. Design decisions
 
diff --git a/docs/performance.md b/docs/performance.md
new file mode 100644
index 0000000..370d2c0
--- /dev/null
+++ b/docs/performance.md
@@ -0,0 +1,78 @@
+# Performance trace, load test and read-ahead
+
+When pages load slowly, the trace shows where the time goes: the storage (a NAS, a disk waking up), the
+reader process (CPU, event loop), library scans running at the same time, or the network to the browser.
+
+## Switching the trace on and off
+
+No restart is needed. The reader checks for a flag file every 5 seconds:
+
+```bash
+touch "$DATA/perf/trace.on"     # on   (Docker: the data folder mounted at /data)
+rm "$DATA/perf/trace.on"        # off
+```
+
+`PERF_TRACE=1` in the environment keeps it on from the start. While it is off, the reader does no extra work.
+
+The trace goes to `DATA/perf/trace-YYYY-MM-DD.jsonl` (UTC days), one JSON object per line. Files older than
+14 days are deleted; one day stops growing at 256 MB. The file contains request paths (manga and chapter
+names) and user names, so treat it like the rest of `DATA`.
+
+## What is recorded
+
+Every line has `t` (server time, ms since the epoch) and `k` (kind):
+
+| `k` | Fields | Meaning |
+|---|---|---|
+| `req` | `m`, `path`, `status`, `ms`, `bytes`, `conn`, `aborted`; for files `stat`, `open`, `first`, `readEnd`, `disk`, `netWait`; for pages also `access` | One HTTP request. Phase fields are ms since the request arrived: `stat` file lookup done, `open` file opened, `first` first chunk read, `readEnd` last chunk read. `disk` = time spent waiting for the storage, `netWait` = time the file stream was paused because the client did not take data fast enough. `conn` > 1 means a reused keep-alive connection |
+| `client` | `e`: `hello`, `res`, `view`, `stall` | Reported by the reader page while the trace is on (`public/perf.js`). `res`: the browser's resource timing of a page image (`queue`, `dns`, `connect`, `ttfb`, `download`, `total`, `size`, `server` = the reader's `stat` time, `cached`). `view`: a page turn in page/book mode, `wait` until the new page was decoded and painted, `preloaded`. `stall`: flow mode, a page on screen before its image, `wait` until it loaded. `hello`: browser, viewport, connection type |
+| `nas` | `statMs`, `firstMs`, `ms`, `bytes` or `error` | Every `PERF_NAS_PROBE_SECONDS` (60) one random library page is read. Out of a large library it is almost never cached, so this is the storage's cold latency over time; seconds after a quiet period mean disks were in standby |
+| `loop` | `lagP50`, `lagP99`, `lagMax`, `cpu`, `rss`, `heap` | Every 5 s: event-loop delay (ms), CPU % of one core, memory (MB) |
+| `scan` | `mode`, `ms`, `changed` | Library scans (change checks every `SCAN_INTERVAL`, full reconciliations) |
+| `ahead` | `tag`, `files`, `done`, `skipped`, `bytes`, `ms`, `replaced` | A read-ahead batch (below) |
+| `scraper` | `endpoint`, `ms`, `status`/`error` | Calls from the reader to the scraper daemon |
+| `trace` | `on` | The trace was switched on or off |
+
+## Reading the result
+
+```bash
+node tools/perf-report.js "$DATA/perf"                          # all days
+node tools/perf-report.js "$DATA/perf" --since 2026-10-29T18:00Z --slow 300
+docker compose exec reader node tools/perf-report.js /data/perf  # inside the container
+```
+
+The report starts with a verdict (which phase dominates the slow loads), then percentiles for the server's
+phases, the browser's timings, page turns, flow stalls, the storage probe, the event loop, scans, read-ahead
+and the API routes. `--json` prints the same numbers for scripts and agents.
+
+How to read it:
+
+- **Slow `firstRead` / `readRest`, slow `nas` probes**: the storage is slow. Probes of several seconds after
+  minutes without reads point at disk standby (on a Synology: *Control Panel → Hardware & Power → HDD
+  Hibernation*).
+- **Slow `stat`**: metadata lookups on the storage are slow (often during a scan, see `scan`).
+- **High `netWait`, or browser `download` much longer than the server's `ms`**: the network between server
+  and browser (Wi-Fi, VPN, a proxy).
+- **Browser `queue`**: the browser waited for a free connection (too many parallel requests).
+- **High `lagMax`**: the reader process was busy and could not answer.
+
+## Load test
+
+`tools/perf-bench.js` simulates readers paging through random chapters with a think time and a small
+look-ahead, and reports how long a reader looked at an empty page (`wait`), time to first byte and total
+per page. It only reads (no progress is written). Use a separate account:
+
+```bash
+BENCH_PASSWORD=… node tools/perf-bench.js --url http://server:8080 --user bench --password-env BENCH_PASSWORD \
+  --readers 4 --chapters 3 --pages 40 --think 400 --ahead 2
+```
+
+Run it with the trace on to see the server's side of the same requests.
+
+## Read-ahead
+
+`READ_AHEAD=1` (default): when a chapter is opened, the reader reads its pages once in page order, starting
+at the saved page, two at a time, and then the next chapter's. The data is thrown away; the OS page cache
+keeps it, so the browser's requests are answered from memory instead of the storage. Opening another
+chapter replaces whatever is still queued. `READ_AHEAD=0` turns it off (useful on a local SSD, or to compare
+with the trace).
diff --git a/perf/read-ahead.js b/perf/read-ahead.js
new file mode 100644
index 0000000..6caa2e7
--- /dev/null
+++ b/perf/read-ahead.js
@@ -0,0 +1,75 @@
+'use strict';
+/*
+ * Read-ahead for network libraries: when a chapter is opened, its pages (then the next chapter's) are read
+ * once in page order, a few at a time, so the OS page cache holds them before the browser asks. On a NAS
+ * with spinning disks a cold page costs 10-100 ms (seconds while a disk wakes up); a cached one well under
+ * 5 ms. The data is discarded; only the cache effect matters.
+ *
+ * Only the newest chapter request counts: opening another chapter replaces whatever is still queued.
+ */
+const fs = require('fs');
+
+class ReadAhead {
+  constructor({ concurrency = 2, remember = 4000, onBatch = () => {} } = {}) {
+    this.concurrency = concurrency; this.remember = remember; this.onBatch = onBatch;
+    this.queue = []; this.active = 0; this.recent = new Map(); this.batch = null; this.buffers = [];
+  }
+
+  // `files` in reading order; `tag` names the batch in the trace.
+  warm(files, tag) {
+    const fresh = files.filter(f => !this.recent.has(f));
+    if (this.batch) this.finishBatch(true);
+    this.queue = fresh;
+    this.batch = { tag, files: fresh.length, skipped: files.length - fresh.length, bytes: 0, t0: performance.now(), done: 0 };
+    if (!fresh.length) this.finishBatch(false);
+    this.pump();
+  }
+  // Runs after the current batch (e.g. the next chapter) unless a newer warm() replaces it first.
+  then(files, tag) {
+    const after = () => this.warm(files, tag);
+    if (!this.batch) after(); else this.batch.next = after;
+  }
+
+  pump() {
+    while (this.active < this.concurrency && this.queue.length) {
+      const file = this.queue.shift(), batch = this.batch;
+      this.active++;
+      this.readFile(file).then(bytes => {
+        this.note(file);
+        if (batch === this.batch) { batch.bytes += bytes; batch.done++; }
+      }, () => {}).finally(() => {
+        this.active--;
+        // A replaced batch's reads may still be running; the current batch ends when all reads have.
+        if (this.batch && !this.queue.length && !this.active) this.finishBatch(false);
+        this.pump();
+      });
+    }
+  }
+  finishBatch(replaced) {
+    const b = this.batch; this.batch = null;
+    if (!b) return;
+    this.onBatch({ tag: b.tag, files: b.files, done: b.done, skipped: b.skipped, bytes: b.bytes,
+      ms: Math.round(performance.now() - b.t0), replaced });
+    if (!replaced && b.next) b.next();
+  }
+  note(file) {
+    this.recent.delete(file); this.recent.set(file, 1);
+    while (this.recent.size > this.remember) this.recent.delete(this.recent.keys().next().value);
+  }
+
+  async readFile(file) {
+    const buf = this.buffers.pop() || Buffer.allocUnsafe(256 * 1024);
+    const fh = await fs.promises.open(file, 'r');
+    let total = 0;
+    try {
+      for (;;) {
+        const { bytesRead } = await fh.read(buf, 0, buf.length, total);
+        if (!bytesRead) break;
+        total += bytesRead;
+      }
+    } finally { await fh.close(); this.buffers.push(buf); }
+    return total;
+  }
+}
+
+module.exports = { ReadAhead };
diff --git a/perf/trace.js b/perf/trace.js
new file mode 100644
index 0000000..f4bd0aa
--- /dev/null
+++ b/perf/trace.js
@@ -0,0 +1,173 @@
+'use strict';
+/*
+ * Switchable performance trace: one JSON object per line in DATA/perf/trace-YYYY-MM-DD.jsonl.
+ *
+ * On while PERF_TRACE=1 or while the file DATA/perf/trace.on exists (checked every few seconds, so it can be
+ * switched on a running server with `touch` / `rm`). Off, every call is a cheap no-op.
+ *
+ * Event kinds (field `k`): `req` one HTTP request with its phases, `client` timings the reader page reports,
+ * `loop` event-loop lag / CPU / memory every SAMPLE_MS, `nas` a periodic read of one uncached library page,
+ * `scan` library scans, `scraper` calls to the scraper daemon, `ahead` read-ahead batches, `trace` on/off.
+ * The format and the report tool: docs/performance.md, tools/perf-report.js.
+ */
+const fs = require('fs'), path = require('path'), { monitorEventLoopDelay } = require('perf_hooks');
+
+const CHECK_MS = 5000, FLUSH_MS = 1000, SAMPLE_MS = 5000;
+const MAX_DAY_BYTES = 256 * 1024 * 1024; // a runaway trace never fills the data disk
+const KEEP_DAYS = 14;
+
+class Tracer {
+  constructor({ dir, env = process.env, now = Date.now } = {}) {
+    this.dir = dir; this.now = now;
+    this.forced = env.PERF_TRACE === '1';
+    this.flag = path.join(dir, 'trace.on');
+    this.on = false; this.buf = []; this.day = ''; this.dayBytes = 0; this.dropped = 0;
+    this.timers = []; this.probe = null; this.probeMs = Math.max(10, +env.PERF_NAS_PROBE_SECONDS || 60) * 1000;
+  }
+
+  start() {
+    this.check();
+    this.timers.push(setInterval(() => this.check(), CHECK_MS).unref());
+    this.timers.push(setInterval(() => this.flush(), FLUSH_MS).unref());
+    return this;
+  }
+  stop() { for (const t of this.timers) clearInterval(t); this.timers = []; this.setOn(false); this.flush(); }
+
+  check() {
+    let on = this.forced;
+    if (!on) try { fs.accessSync(this.flag); on = true; } catch { /* off */ }
+    this.setOn(on);
+  }
+  setOn(on) {
+    if (on === this.on) return;
+    if (on) {
+      this.on = true;
+      this.log({ k: 'trace', on: true, pid: process.pid, node: process.version });
+      this.startSampler();
+    } else {
+      this.log({ k: 'trace', on: false });
+      this.on = false;
+      this.stopSampler();
+    }
+  }
+
+  log(event) {
+    if (!this.on) return;
+    this.buf.push(JSON.stringify({ t: this.now(), ...event }));
+    if (this.buf.length >= 500) this.flush();
+  }
+
+  flush() {
+    if (!this.buf.length) return;
+    const lines = this.buf.join('\n') + '\n'; this.buf = [];
+    const day = new Date(this.now()).toISOString().slice(0, 10);
+    try {
+      if (day !== this.day) { this.day = day; this.dayBytes = 0; fs.mkdirSync(this.dir, { recursive: true }); this.prune(); }
+      if (this.dayBytes + lines.length > MAX_DAY_BYTES) { this.dropped += lines.length; return; }
+      fs.appendFileSync(path.join(this.dir, `trace-${day}.jsonl`), lines);
+      this.dayBytes += lines.length;
+    } catch (error) { console.warn('[perf] trace write failed:', error.message); }
+  }
+  prune() {
+    const cutoff = new Date(this.now() - KEEP_DAYS * 864e5).toISOString().slice(0, 10);
+    for (const name of fs.readdirSync(this.dir)) {
+      const m = /^trace-(\d{4}-\d\d-\d\d)\.jsonl$/.exec(name);
+      if (m && m[1] < cutoff) fs.rmSync(path.join(this.dir, name), { force: true });
+    }
+  }
+
+  // Event-loop delay shows whether the reader itself (CPU, sync work) holds requests back.
+  startSampler() {
+    const h = monitorEventLoopDelay({ resolution: 10 }); h.enable();
+    let cpu = process.cpuUsage(), at = process.hrtime.bigint();
+    this.sampler = { h, timer: setInterval(() => {
+      const nowCpu = process.cpuUsage(), nowAt = process.hrtime.bigint(), wall = Number(nowAt - at) / 1000;
+      const mem = process.memoryUsage();
+      this.log({ k: 'loop', lagP50: ms(h.percentile(50)), lagP99: ms(h.percentile(99)), lagMax: ms(h.max),
+        cpu: round(((nowCpu.user - cpu.user) + (nowCpu.system - cpu.system)) / wall * 100), rss: mb(mem.rss), heap: mb(mem.heapUsed) });
+      h.reset(); cpu = nowCpu; at = nowAt;
+    }, SAMPLE_MS).unref() };
+    this.startProbe();
+  }
+  stopSampler() {
+    if (this.sampler) { clearInterval(this.sampler.timer); this.sampler.h.disable(); this.sampler = null; }
+    if (this.probe) { clearInterval(this.probe); this.probe = null; }
+  }
+
+  // NAS probe: read one random library page now and then. A page picked at random out of a large library is
+  // almost never in the OS cache, so this measures the storage itself (and shows disks waking from standby).
+  setProbeSource(pick) { this.pick = pick; if (this.on) this.startProbe(); }
+  startProbe() {
+    if (this.probe || !this.pick) return;
+    const run = () => this.probeOnce().catch(() => {});
+    this.probe = setInterval(run, this.probeMs).unref();
+    setTimeout(run, 1000).unref();
+  }
+  async probeOnce() {
+    const file = this.pick && this.pick();
+    if (!file) return;
+    const t0 = performance.now();
+    let st, bytes = 0, firstMs = null, error = null;
+    try {
+      st = await fs.promises.stat(file);
+      const statMs = performance.now() - t0;
+      const fh = await fs.promises.open(file, 'r');
+      try {
+        const chunk = Buffer.allocUnsafe(256 * 1024);
+        for (;;) {
+          const { bytesRead } = await fh.read(chunk, 0, chunk.length, bytes);
+          if (firstMs === null) firstMs = performance.now() - t0;
+          if (!bytesRead) break;
+          bytes += bytesRead;
+        }
+      } finally { await fh.close(); }
+      this.log({ k: 'nas', statMs: round(statMs), firstMs: round(firstMs), ms: round(performance.now() - t0), bytes });
+    } catch (e) { error = e.code || e.message; this.log({ k: 'nas', ms: round(performance.now() - t0), error }); }
+  }
+}
+
+// Wraps one HTTP request: `mark(name)` stores the time since the request arrived, `set` adds fields, and the
+// event is written when the response is done (or the client went away). Off, returns a shared no-op.
+const NOOP = { mark() {}, set() {}, stream() {}, elapsed: () => 0, on: false };
+function requestTrace(tracer, req, res) {
+  if (!tracer.on) return NOOP;
+  const t0 = performance.now(), ev = { k: 'req', m: req.method, path: req.url.split('?')[0].slice(0, 300) };
+  const trace = {
+    on: true,
+    mark(name) { ev[name] = round(performance.now() - t0); },
+    elapsed: () => round(performance.now() - t0),
+    set(fields) { Object.assign(ev, fields); },
+    // Splits a file stream's time into waiting for the disk and waiting for the client (TCP backpressure):
+    // pipe() pauses the file stream while the socket buffer is full and resumes it on 'drain'.
+    stream(src) {
+      let pausedAt = 0, waitNet = 0;
+      src.on('open', () => trace.mark('open'));
+      src.once('data', () => trace.mark('first'));
+      src.on('data', chunk => { ev.bytes = (ev.bytes || 0) + chunk.length; });
+      src.on('pause', () => { pausedAt = performance.now(); });
+      src.on('resume', () => { if (pausedAt) { waitNet += performance.now() - pausedAt; pausedAt = 0; } });
+      src.on('end', () => {
+        trace.mark('readEnd');
+        ev.netWait = round(waitNet);
+        ev.disk = round(Math.max(0, ev.readEnd - (ev.open || 0) - waitNet));
+      });
+    },
+  };
+  let done = false;
+  const finish = aborted => {
+    if (done) return; done = true;
+    ev.ms = round(performance.now() - t0); ev.status = res.statusCode;
+    if (aborted) ev.aborted = true;
+    ev.conn = req.socket._perfRequests = (req.socket._perfRequests || 0) + 1; // >1: keep-alive reuse
+    tracer.log(ev);
+  };
+  res.on('finish', () => finish(false));
+  res.on('close', () => finish(!res.writableFinished));
+  return trace;
+}
+
+const round = v => (v == null || Number.isNaN(v) ? null : Math.round(v * 10) / 10);
+const ms = ns => round(ns / 1e6);
+const mb = b => Math.round(b / 1048576);
+
+module.exports = { Tracer, requestTrace };
diff --git a/public/perf.js b/public/perf.js
new file mode 100644
index 0000000..ecbfa12
--- /dev/null
+++ b/public/perf.js
@@ -0,0 +1,72 @@
+'use strict';
+/*
+ * Reader-side half of the performance trace (docs/performance.md). Only started when the server says its
+ * trace is on (`perf: true` in /api/chapter). It reports, in batches to POST api/perf:
+ *  - `res`  the browser's resource timing of every library image: queueing, connection, time to first byte,
+ *           download, size, the server's own `stat` time and whether the connection was reused;
+ *  - `view` a page turn (page/book mode): how long the new page(s) took to appear, and whether they were
+ *           preloaded already;
+ *  - `stall` flow mode: a page scrolled into view before its image had loaded, and how long it stayed empty.
+ */
+window.PerfTrace = {
+  start(pageName) {
+    let queue = [], on = true;
+    const push = e => { if (on) queue.push(e); };
+    const rel = url => { try { return decodeURIComponent(new URL(url).pathname); } catch { return String(url); } };
+    const flush = beacon => {
+      if (!queue.length || !on) return;
+      const body = JSON.stringify({ page: pageName, events: queue.splice(0, 300) });
+      if (beacon && navigator.sendBeacon) navigator.sendBeacon('api/perf', new Blob([body], { type: 'application/json' }));
+      else fetch('api/perf', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body, keepalive: true })
+        .then(r => r.json()).then(r => { if (r.on === false) on = false; }).catch(() => {});
+    };
+    setInterval(() => flush(false), 5000);
+    addEventListener('pagehide', () => flush(true));
+    push({ e: 'hello', ua: navigator.userAgent, conn: navigator.connection ? navigator.connection.effectiveType : '',
+      dpr: devicePixelRatio, vw: innerWidth, vh: innerHeight });
+
+    if (window.PerformanceObserver) {
+      new PerformanceObserver(list => {
+        for (const r of list.getEntries()) {
+          if (!/\/lib\//.test(r.name)) continue;
+          const st = (r.serverTiming || []).find(x => x.name === 'stat');
+          push({ e: 'res', url: rel(r.name), at: r.startTime,
+            queue: r.requestStart ? r.requestStart - r.startTime : null,        // waiting for a free connection
+            connect: r.connectEnd - r.connectStart, dns: r.domainLookupEnd - r.domainLookupStart,
+            ttfb: r.responseStart && r.requestStart ? r.responseStart - r.requestStart : null,
+            download: r.responseStart ? r.responseEnd - r.responseStart : null,
+            total: r.duration, size: r.transferSize, body: r.encodedBodySize, server: st ? st.duration : null,
+            proto: r.nextHopProtocol || '', cached: r.transferSize === 0 && r.encodedBodySize > 0 });
+        }
+      }).observe({ type: 'resource', buffered: true });
+    }
+
+    return {
+      // Called right after a page turn put new <img> elements on screen.
+      view(imgs, info) {
+        const t0 = performance.now(), waiting = imgs.filter(img => !img.complete);
+        const event = { e: 'view', ...info, n: imgs.length, preloaded: !waiting.length };
+        if (!waiting.length) { push({ ...event, wait: 0 }); return; }
+        Promise.all(waiting.map(img => (img.decode ? img.decode() : Promise.resolve()).catch(() => {})))
+          .then(() => requestAnimationFrame(() => push({ ...event, wait: performance.now() - t0,
+            url: rel(waiting[0].currentSrc || waiting[0].src) })));
+      },
+      // Flow mode: watch pages enter the real viewport (not the preload margin) before their image is there.
+      watchFlow(root, els) {
+        if (!window.IntersectionObserver) return;
+        const io = new IntersectionObserver(entries => {
+          for (const en of entries) {
+            const img = en.target.firstChild;
+            if (!en.isIntersecting || (img.complete && img.naturalWidth)) continue;
+            const t0 = performance.now(), hadSrc = !!img.getAttribute('src');
+            const done = () => push({ e: 'stall', wait: performance.now() - t0, hadSrc, url: rel(img.currentSrc || img.src) });
+            img.addEventListener('load', done, { once: true });
+            io.unobserve(en.target);
+          }
+        }, { root });
+        for (const el of els) io.observe(el);
+        return io;
+      },
+    };
+  },
+};
diff --git a/public/read.html b/public/read.html
index f3c0ce5..b282e85 100644
--- a/public/read.html
+++ b/public/read.html
@@ -16,6 +16,7 @@
 <script src="reading-clock.js" defer></script>
 <script src="keys.js" defer></script>
 <script src="moments.js" defer></script>
+<script src="perf.js" defer></script>
 <script src="reader.js" defer></script>
 </head>
 <body class="reader">
diff --git a/public/reader.js b/public/reader.js
index 81dbfc1..4971301 100644
--- a/public/reader.js
+++ b/public/reader.js
@@ -74,6 +74,9 @@
     D = payload;
   }
   catch (e) { canvas.textContent = ''; canvas.append(h('p', { class: 'loading' }, e.message + ' · ', h('a', { href: './' }, 'Library'))); return; }
+  // Timings for the server's performance trace, only while an admin has it switched on.
+  const perf = D.perf && window.PerfTrace ? PerfTrace.start('read') : null;
+  let perfFlow = null;
 
   // An already-open chapter also closes when a window ends or an unlock expires.
   const checkReadingAccess = async () => {
@@ -250,11 +253,13 @@
     if (mode === 'flow') {
       els = list.map(p => canvas.appendChild(pageEl(p, true)));
       endEl = canvas.appendChild(endCard());
+      if (perf) { if (perfFlow) perfFlow.disconnect(); perfFlow = perf.watchFlow(stage, els); }
     } else if (atEnd) {
       endEl = canvas.appendChild(endCard());
     } else {
       shown = mode === 'page' ? [cur] : spreads[spreadIdx];
       els = shown.map(k => canvas.appendChild(pageEl(list[k])));
+      if (perf) perf.view(els.map(el => el.firstChild), { mode, page: list[shown[0]].i });
       // The pages on screen get the bandwidth first, then the next ones are fetched.
       const token = ++buildToken, waiting = els.map(el => el.firstChild).filter(img => !img.complete);
       Promise.all(waiting.map(img => new Promise(done => {
diff --git a/server.js b/server.js
index 3667a64..bce53ee 100644
--- a/server.js
+++ b/server.js
@@ -12,6 +12,8 @@
  *   SCRAPER_URL=http://127.0.0.1:8091   scraper daemon API
  *   SCRAPER_TOKEN=...                    shared daemon token (optional on loopback)
  *   BACKUP=/backup       folder for built-in DATA backups (unset = backups off)
+ *   READ_AHEAD=1         read an opened chapter's pages (then the next chapter's) into the OS cache, 0 = off
+ *   PERF_TRACE=1         performance trace to DATA/perf/ (or touch DATA/perf/trace.on at runtime; docs/performance.md)
  */
 const http = require('http'), fs = require('fs'), path = require('path'), crypto = require('crypto');
 const { scanLibrary, scanChangedLibrary, libraryStamps, snapshotIndex, restoreIndex } = require('./library');
@@ -36,6 +38,8 @@ const { chapterDates } = require('./chapter-dates');
 const { parseSchedule, publicSchedule, loadQuestionBank, createReadingAccess } = require('./reading-access');
 const { BackupService, BackupError } = require('./backup');
 const moments = require('./moments');
+const { Tracer, requestTrace } = require('./perf/trace');
+const { ReadAhead } = require('./perf/read-ahead');
 
 const PORT = +process.env.PORT || 8080;
 const LIB = path.resolve(process.env.LIBRARY || 'library');
@@ -64,6 +68,9 @@ const MIME = {
 
 // ---------- storage: one JSON file, written atomically ----------
 fs.mkdirSync(DATA, { recursive: true });
+const tracer = new Tracer({ dir: path.join(DATA, 'perf') }).start();
+const readAhead = process.env.READ_AHEAD === '0' ? null
+  : new ReadAhead({ onBatch: batch => tracer.log({ k: 'ahead', ...batch }) });
 const catalog = new Catalog(DATA);
 const greatestCatalog = loadGreatestCatalog();
 const greatestById = new Map(greatestCatalog.entries.map(entry => [entry.id, entry]));
@@ -175,8 +182,10 @@ async function performScan(mode) {
       scanState = { at: Date.now(), ms: Date.now() - t0, mode, changedMangas: changed.length,
         mangas: lib.mangas.length, chapters, pages, stale: false, error: null };
       console.log(`[scan] ${mode}: ${changed.length} changed manga, ${lib.mangas.length} manga, ${chapters} chapters, ${pages} pages in ${scanState.ms} ms`);
+      tracer.log({ k: 'scan', mode, ms: scanState.ms, changed: changed.length });
       return scanState;
     } catch (e) {
+      tracer.log({ k: 'scan', mode, ms: Date.now() - t0, error: e.message });
       scanState = { ...scanState, at: Date.now(), error: e.message };
       console.error('[scan] failed:', e.message);
       return scanState;
@@ -253,9 +262,14 @@ async function scraperRequest(endpoint, body) {
     options.method = 'POST'; headers['content-type'] = 'application/json'; options.body = JSON.stringify(body);
   }
   let response;
+  const t0 = performance.now();
   try { response = await fetch(SCRAPER_URL + endpoint, options); }
-  catch (error) { throw new HttpError(503, `Scraper service unavailable: ${error.message}`); }
+  catch (error) {
+    tracer.log({ k: 'scraper', endpoint, ms: Math.round(performance.now() - t0), error: error.message });
+    throw new HttpError(503, `Scraper service unavailable: ${error.message}`);
+  }
   const data = await response.json().catch(() => ({}));
+  tracer.log({ k: 'scraper', endpoint, ms: Math.round(performance.now() - t0), status: response.status });
   if (!response.ok) throw new HttpError(response.status >= 500 ? 502 : response.status, data.error || `Scraper HTTP ${response.status}`);
   return data;
 }
@@ -433,6 +447,17 @@ function continueOf(m, prog, s) {
   const next = m.chapters.slice(m.chapters.indexOf(last.ch) + 1).find(ok);
   return next ? { c: next.id, text: 'Read next · ' + next.label } : { c: last.ch.id, v: last.p.v, text: 'Read again · ' + last.ch.label };
 }
+// From the saved page on, then the pages before it; afterwards the next chapter's preferred version.
+function warmChapter(m, ch, v, progress, s) {
+  const from = Math.min(Math.max(0, +(progress && progress.v === v.id && progress.p) || 0), v.pages.length);
+  const pages = v.pages.slice(from).concat(v.pages.slice(0, from)).map(p => path.join(LIB, v.rel, p.f));
+  readAhead.warm(pages, `${m.id}/${ch.id}`);
+  const next = neighbour(m, ch, 1, s), nextCh = next && m.chapterById.get(next.id);
+  if (nextCh) {
+    const nv = ranked(m, nextCh)[0].v;
+    readAhead.then(nv.pages.map(p => path.join(LIB, nv.rel, p.f)), `${m.id}/${nextCh.id}`);
+  }
+}
 function neighbour(m, ch, step, s) {
   for (let i = m.chapters.indexOf(ch) + step; i >= 0 && i < m.chapters.length; i += step) {
     const c = m.chapters[i];
@@ -743,7 +768,9 @@ const routes = {
     let seen = false;
     for (const n of db.notes[u.name] || []) if (!n.read && n.m === m.id && n.c === ch.id) seen = n.read = true;
     if (seen) save();
+    if (readAhead) warmChapter(m, ch, v, prog[ch.id], s);
     return {
+      perf: tracer.on,
       manga: { id: m.id, title: m.title },
       chapter: { id: ch.id, label: ch.label, title: ch.title, extra: ch.extra },
       version: v.id, versions: versionsOf(m, ch, u), direction: v.direction,
@@ -875,6 +902,21 @@ const routes = {
     return { ok: true };
   },
 
+  // Timings the reader page measured (resource timing, decode, page-turn waits) while the trace is on.
+  'POST /api/perf': ({ u, b }) => {
+    if (!tracer.on) return { on: false };
+    const events = Array.isArray(b.events) ? b.events.slice(0, 300) : [];
+    for (const e of events) if (e && typeof e === 'object' && !Array.isArray(e)) {
+      const clean = {};
+      for (const [key, value] of Object.entries(e).slice(0, 30)) {
+        if (key === 't' || key === 'k') continue; // the server's clock and kind
+        if (typeof value === 'number' && Number.isFinite(value)) clean[key] = Math.round(value * 10) / 10;
+        else if (typeof value === 'string' || typeof value === 'boolean') clean[key] = typeof value === 'string' ? value.slice(0, 300) : value;
+      }
+      tracer.log({ ...clean, k: 'client', user: u.name, page: b.page ? String(b.page).slice(0, 40) : undefined });
+    }
+    return { on: true };
+  },
   'GET /api/notifications': ({ u }) => ({ items: (db.notes[u.name] || []).slice().reverse() }),
   'POST /api/notifications/read': ({ u, b }) => {
     for (const n of db.notes[u.name] || []) if (b.all || n.id === b.id) n.read = true;
@@ -1062,21 +1104,28 @@ async function handleApi(req, res, u, route, q) {
   }
   sendJson(res, 200, await route({ req, res, u, b, q }));
 }
-function serveFile(req, res, root, rel, cacheControl, only) {
+function serveFile(req, res, root, rel, cacheControl, only, trace) {
   let file;
   try { file = path.resolve(root, decodeURIComponent(rel)); } catch { return sendJson(res, 400, { error: 'Bad path' }); }
   if (!file.startsWith(root + path.sep) || (only && !only.test(file))) return sendJson(res, 404, { error: 'Not found' });
   fs.stat(file, (err, st) => {
+    if (trace) trace.mark('stat');
     if (err || !st.isFile()) return sendJson(res, 404, { error: 'Not found' });
     const etag = `"${st.size.toString(36)}-${Math.round(st.mtimeMs).toString(36)}"`;
     if (req.headers['if-none-match'] === etag) { res.writeHead(304, { ETag: etag, 'Cache-Control': cacheControl }); return res.end(); }
-    res.writeHead(200, { 'Content-Type': MIME[path.extname(file).toLowerCase()] || 'application/octet-stream', 'Content-Length': st.size, ETag: etag, 'Cache-Control': cacheControl });
+    const headers = { 'Content-Type': MIME[path.extname(file).toLowerCase()] || 'application/octet-stream', 'Content-Length': st.size, ETag: etag, 'Cache-Control': cacheControl };
+    // The browser's resource timing shows this, so a slow page can be matched to the server's share.
+    if (trace && trace.on) headers['Server-Timing'] = `stat;dur=${trace.elapsed()}`;
+    res.writeHead(200, headers);
     if (req.method === 'HEAD') return res.end();
-    fs.createReadStream(file).on('error', () => res.destroy()).pipe(res);
+    const stream = fs.createReadStream(file).on('error', () => res.destroy());
+    if (trace) trace.stream(stream);
+    stream.pipe(res);
   });
 }
 
 http.createServer(async (req, res) => {
+  const trace = requestTrace(tracer, req, res);
   try {
     const { pathname, searchParams } = new URL(req.url, 'http://localhost');
     const u = userOf(req);
@@ -1124,9 +1173,10 @@ http.createServer(async (req, res) => {
       const parts = relative.split(path.sep);
       if (parts.length >= 4 && !readingAccess.status(u.name).allowed)
         bad(423, 'Reading is outside the deployment schedule.');
-      return serveFile(req, res, LIB, pathname.slice(5), readingSchedule && parts.length >= 4 ? 'private, no-store' : 'private, max-age=86400', IMG);
+      trace.mark('access');
+      return serveFile(req, res, LIB, pathname.slice(5), readingSchedule && parts.length >= 4 ? 'private, no-store' : 'private, max-age=86400', IMG, trace);
     }
-    serveFile(req, res, PUB, pathname === '/' ? 'index.html' : pathname.slice(1), 'no-cache');
+    serveFile(req, res, PUB, pathname === '/' ? 'index.html' : pathname.slice(1), 'no-cache', null, trace);
   } catch (e) {
     const expected = e instanceof HttpError || e instanceof CatalogError || e instanceof moments.MomentError;
     if (!expected) console.error(e);
@@ -1134,6 +1184,13 @@ http.createServer(async (req, res) => {
   }
 }).listen(PORT, () => console.log(`MangaCircus on http://localhost:${PORT}  (library: ${LIB}, data: ${DATA})`));
 
+// The trace's storage probe reads a random page of the library (rarely in the OS cache).
+tracer.setProbeSource(() => {
+  const m = lib.mangas[Math.floor(Math.random() * lib.mangas.length)];
+  const ch = m && m.chapters[Math.floor(Math.random() * m.chapters.length)], v = ch && ch.versions[0];
+  const p = v && v.pages[Math.floor(Math.random() * v.pages.length)];
+  return p ? path.join(LIB, v.rel, p.f) : null;
+});
 if (lib.mangas.length) refreshChanged();
 else rescan();
 if (SCAN_MINUTES > 0) setInterval(() => refreshChanged(), SCAN_MINUTES * 60000).unref();
diff --git a/test/dockerfile.test.js b/test/dockerfile.test.js
index dbc41e3..dfe2d83 100644
--- a/test/dockerfile.test.js
+++ b/test/dockerfile.test.js
@@ -3,6 +3,7 @@
 const test = require('node:test'), assert = require('node:assert/strict');
 const fs = require('fs'), path = require('path');
 const root = path.resolve(__dirname, '..');
+const allowedNames = () => new Set(fs.readFileSync(path.join(root, '.dockerignore'), 'utf8').split(/\r?\n/).filter(line => line.startsWith('!')).map(line => line.slice(1)));
 
 test('every top-level module the reader requires is copied into the image', () => {
   const dockerfile = fs.readFileSync(path.join(root, 'Dockerfile'), 'utf8');
@@ -12,14 +13,19 @@ test('every top-level module the reader requires is copied into the image', () =
     const file = pending.pop();
     if (seen.has(file)) continue;
     seen.add(file);
-    for (const [, name] of fs.readFileSync(path.join(root, file), 'utf8').matchAll(/require\('\.\/([\w.-]+?)(?:\.js)?'\)/g)) {
+    const source = fs.readFileSync(path.join(root, file), 'utf8');
+    for (const [, dir] of source.matchAll(/require\('\.\/([\w.-]+)\/[^']+'\)/g)) {
+      assert.ok(copied.has(dir), `${dir}/ is not copied`);
+      assert.ok(allowedNames().has(dir + '/'), `${dir}/ is not allowed through .dockerignore`);
+    }
+    for (const [, name] of source.matchAll(/require\('\.\/([\w.-]+?)(?:\.js)?'\)/g)) {
       // A folder (e.g. ./greatest) is copied as a whole; only single files need their own name in COPY.
       if (fs.existsSync(path.join(root, name)) && fs.statSync(path.join(root, name)).isDirectory()) assert.ok(copied.has(name), `${name}/ is not copied`);
       else pending.push(name + '.js');
     }
   }
   // .dockerignore is an allowlist too: a file not let through never reaches COPY.
-  const allowed = new Set(fs.readFileSync(path.join(root, '.dockerignore'), 'utf8').split(/\r?\n/).filter(line => line.startsWith('!')).map(line => line.slice(1)));
+  const allowed = allowedNames();
   for (const file of seen) {
     assert.ok(copied.has(file), `${file} is required but not copied by the Dockerfile`);
     assert.ok(allowed.has(file), `${file} is required but not allowed through .dockerignore`);
diff --git a/test/perf.test.js b/test/perf.test.js
new file mode 100644
index 0000000..0dd46ef
--- /dev/null
+++ b/test/perf.test.js
@@ -0,0 +1,104 @@
+'use strict';
+const test = require('node:test'), assert = require('node:assert/strict');
+const fs = require('fs'), os = require('os'), path = require('path'), http = require('http');
+const { Tracer, requestTrace } = require('../perf/trace');
+const { ReadAhead } = require('../perf/read-ahead');
+const { analyse } = require('../tools/perf-report');
+
+const tmp = () => fs.mkdtempSync(path.join(os.tmpdir(), 'circus-perf-'));
+const lines = dir => fs.readdirSync(dir).filter(n => n.endsWith('.jsonl'))
+  .flatMap(n => fs.readFileSync(path.join(dir, n), 'utf8').trim().split('\n')).filter(Boolean).map(JSON.parse);
+
+test('tracer is off without the flag file and writes nothing', () => {
+  const dir = tmp(), tracer = new Tracer({ dir, env: {} });
+  tracer.check(); tracer.log({ k: 'x' }); tracer.flush();
+  assert.equal(tracer.on, false);
+  assert.deepEqual(fs.readdirSync(dir), []);
+});
+
+test('the flag file switches the trace on and off at runtime', () => {
+  const dir = tmp(), tracer = new Tracer({ dir, env: {} });
+  fs.writeFileSync(path.join(dir, 'trace.on'), '');
+  tracer.check(); tracer.log({ k: 'x', a: 1 });
+  fs.rmSync(path.join(dir, 'trace.on'));
+  tracer.check(); tracer.log({ k: 'x', a: 2 }); tracer.flush();
+  const events = lines(dir);
+  assert.deepEqual(events.map(e => e.k), ['trace', 'x', 'trace']);
+  assert.equal(events[1].a, 1);
+  assert.equal(events[2].on, false);
+  assert.ok(events.every(e => typeof e.t === 'number'));
+});
+
+test('a served file is traced with storage and network phases', async () => {
+  const dir = tmp(), tracer = new Tracer({ dir, env: { PERF_TRACE: '1' } });
+  tracer.check();
+  const file = path.join(dir, 'page.jpg');
+  fs.writeFileSync(file, Buffer.alloc(3 * 1024 * 1024, 7));
+  const server = http.createServer((req, res) => {
+    const trace = requestTrace(tracer, req, res);
+    fs.stat(file, (err, st) => {
+      trace.mark('stat');
+      res.writeHead(200, { 'Content-Length': st.size });
+      const stream = fs.createReadStream(file);
+      trace.stream(stream);
+      stream.pipe(res);
+    });
+  });
+  await new Promise(r => server.listen(0, r));
+  const { port } = server.address();
+  const r = await fetch(`http://127.0.0.1:${port}/lib/M/B/C/page.jpg`);
+  assert.equal((await r.arrayBuffer()).byteLength, 3 * 1024 * 1024);
+  await new Promise(r => setTimeout(r, 50));
+  server.close(); tracer.stopSampler(); tracer.flush();
+  const req = lines(dir).find(e => e.k === 'req');
+  assert.equal(req.status, 200);
+  assert.equal(req.bytes, 3 * 1024 * 1024);
+  assert.equal(req.path, '/lib/M/B/C/page.jpg');
+  for (const field of ['stat', 'open', 'first', 'readEnd', 'disk', 'netWait', 'ms']) assert.equal(typeof req[field], 'number', field);
+  assert.ok(req.ms >= req.readEnd && req.readEnd >= req.first && req.first >= req.open);
+});
+
+test('read-ahead reads files in order and a newer chapter replaces the queued one', async () => {
+  const dir = tmp(), files = [];
+  for (let i = 0; i < 6; i++) { const f = path.join(dir, `p${i}.jpg`); fs.writeFileSync(f, Buffer.alloc(1000 + i)); files.push(f); }
+  const batches = [];
+  const ahead = new ReadAhead({ concurrency: 1, onBatch: b => batches.push(b) });
+  const done = new Promise(resolve => { const check = () => (batches.some(b => b.tag === 'B' && !b.replaced) ? resolve() : setTimeout(check, 5)); check(); });
+  ahead.warm(files.slice(0, 4), 'A');
+  ahead.warm(files.slice(3), 'B');
+  await done;
+  assert.equal(batches[0].tag, 'A'); assert.equal(batches[0].replaced, true);
+  const b = batches.find(x => x.tag === 'B');
+  assert.equal(b.done, 3); assert.equal(b.bytes, 1003 + 1004 + 1005);
+  // Files read recently are skipped the next time.
+  batches.length = 0;
+  ahead.warm(files.slice(4), 'C');
+  assert.deepEqual(batches.map(x => [x.tag, x.files, x.skipped]), [['C', 0, 2]]);
+});
+
+test('read-ahead runs the follow-up batch after the current one', async () => {
+  const dir = tmp(), a = path.join(dir, 'a.jpg'), b = path.join(dir, 'b.jpg');
+  fs.writeFileSync(a, 'a'); fs.writeFileSync(b, 'bb');
+  const tags = [];
+  const ahead = new ReadAhead({ onBatch: x => tags.push(x.tag) });
+  ahead.warm([a], 'now'); ahead.then([b], 'next');
+  await new Promise(r => { const check = () => (tags.length === 2 ? r() : setTimeout(check, 5)); check(); });
+  assert.deepEqual(tags, ['now', 'next']);
+});
+
+test('the report blames the storage for slow first reads', () => {
+  const t = Date.parse('2026-10-29T20:00:00Z');
+  const events = [
+    { t, k: 'req', m: 'GET', path: '/lib/M/B/C/1.jpg', status: 200, bytes: 300000, stat: 2, open: 3, first: 1900, readEnd: 1950, disk: 1947, netWait: 0, ms: 1955, conn: 1 },
+    { t: t + 1000, k: 'req', m: 'GET', path: '/lib/M/B/C/2.jpg', status: 200, bytes: 300000, stat: 1, open: 2, first: 5, readEnd: 20, disk: 18, netWait: 0, ms: 22, conn: 2 },
+    { t: t + 2000, k: 'client', e: 'res', url: '/lib/M/B/C/1.jpg', total: 2100, ttfb: 30, download: 2000, queue: 1, connect: 0, dns: 0 },
+    { t: t + 3000, k: 'client', e: 'view', wait: 2050, preloaded: false },
+    { t: t + 4000, k: 'nas', firstMs: 2500, ms: 2600, bytes: 1 },
+  ];
+  const o = analyse(events, { slow: 500 });
+  assert.equal(o.slowServer.count, 1);
+  assert.deepEqual(o.slowServer.by, { firstRead: 1 });
+  assert.deepEqual(o.slowBrowser.by, { server: 1 });
+  assert.equal(o.pageTurns.over1s, 1);
+  assert.match(o.verdict.join('\n'), /first read from storage/);
+});
diff --git a/tools/perf-bench.js b/tools/perf-bench.js
new file mode 100644
index 0000000..95c6d30
--- /dev/null
+++ b/tools/perf-bench.js
@@ -0,0 +1,110 @@
+#!/usr/bin/env node
+'use strict';
+/*
+ * Load test for the reader: N simulated readers each open random chapters and fetch the pages one after
+ * another with a think time, the way a person paging through a chapter does, plus a small look-ahead.
+ * Measures time to first byte and total per page from this machine. Read-only apart from the login; it
+ * never writes progress. Run it with the server's trace on to see the server's share (docs/performance.md).
+ *
+ *   node tools/perf-bench.js --url http://host:8080 --user bench --password-env BENCH_PASSWORD \
+ *        [--readers 4] [--chapters 3] [--pages 40] [--think 400] [--ahead 2] [--manga Berserk] [--json]
+ *   --cookie 'sid=…' instead of user/password.  --chapter-only skips the page fetches (API timings only).
+ */
+
+function parseArgs(argv) {
+  const o = { url: 'http://localhost:8080', readers: 4, chapters: 3, pages: 40, think: 400, ahead: 2, manga: '', json: false };
+  for (let i = 0; i < argv.length; i++) {
+    const key = argv[i].replace(/^--/, ''), next = () => argv[++i];
+    if (['readers', 'chapters', 'pages', 'think', 'ahead'].includes(key)) o[key] = +next();
+    else if (key === 'password-env') o.password = process.env[next()] || '';
+    else if (['url', 'user', 'cookie', 'manga'].includes(key)) o[key] = next();
+    else if (key === 'json' || key === 'chapter-only') o[key.replace('-o', 'O')] = true;
+    else if (key === 'h' || key === 'help') o.help = true;
+  }
+  o.url = o.url.replace(/\/$/, '');
+  return o;
+}
+
+async function login(o) {
+  if (o.cookie) return o.cookie;
+  const r = await fetch(o.url + '/api/login', { method: 'POST', headers: { 'content-type': 'application/json' },
+    body: JSON.stringify({ name: o.user, password: o.password }) });
+  if (!r.ok) throw new Error(`login failed: HTTP ${r.status} ${await r.text()}`);
+  const cookie = (r.headers.get('set-cookie') || '').split(';')[0];
+  if (!cookie) throw new Error('login returned no session cookie');
+  return cookie;
+}
+
+async function timed(url, cookie) {
+  const t0 = performance.now();
+  const r = await fetch(url, { headers: { cookie } });
+  const ttfb = performance.now() - t0;
+  let bytes = 0;
+  for await (const chunk of r.body) bytes += chunk.length;
+  return { status: r.status, ttfb, ms: performance.now() - t0, bytes, server: r.headers.get('server-timing') || '' };
+}
+const json = async (o, cookie, path) => {
+  const t0 = performance.now(), r = await fetch(o.url + path, { headers: { cookie } });
+  if (!r.ok) throw new Error(`${path}: HTTP ${r.status}`);
+  return { data: await r.json(), ms: performance.now() - t0 };
+};
+const pick = list => list[Math.floor(Math.random() * list.length)];
+const sleep = ms => new Promise(r => setTimeout(r, ms));
+
+async function reader(o, cookie, mangas, out, id) {
+  for (let c = 0; c < o.chapters; c++) {
+    const m = pick(mangas);
+    const manga = await json(o, cookie, '/api/manga?m=' + encodeURIComponent(m.id));
+    out.api.push({ path: '/api/manga', ms: manga.ms });
+    const ch = pick(manga.data.chapters);
+    const chapter = await json(o, cookie, `/api/chapter?m=${encodeURIComponent(m.id)}&c=${encodeURIComponent(ch.id)}`);
+    out.api.push({ path: '/api/chapter', ms: chapter.ms });
+    if (o.chapterOnly) continue;
+    const pages = chapter.data.pages.slice(0, o.pages);
+    // The browser keeps `ahead` pages in flight while the reader looks at the current one.
+    const inflight = new Map();
+    const start = k => { if (k < pages.length && !inflight.has(k)) inflight.set(k, timed(o.url + '/' + pages[k].u, cookie)); };
+    for (let k = 0; k < pages.length; k++) {
+      const waitFrom = performance.now();
+      start(k);
+      for (let a = 1; a <= o.ahead; a++) start(k + a);
+      const r = await inflight.get(k);
+      out.pages.push({ reader: id, manga: m.id, chapter: ch.id, page: k, wait: performance.now() - waitFrom, ...r });
+      await sleep(o.think * (0.5 + Math.random()));
+    }
+    await Promise.all(inflight.values());
+  }
+}
+
+const pct = (v, p) => { const s = v.slice().sort((a, b) => a - b); return s.length ? Math.round(s[Math.min(s.length - 1, Math.floor(p / 100 * s.length))]) : null; };
+const sum = v => ({ n: v.length, p50: pct(v, 50), p90: pct(v, 90), p99: pct(v, 99), max: v.length ? Math.round(Math.max(...v)) : null });
+
+async function main() {
+  const o = parseArgs(process.argv.slice(2));
+  if (o.help || (!o.cookie && !o.user)) {
+    console.log('usage: node tools/perf-bench.js --url URL (--user NAME --password-env VAR | --cookie sid=…) [--readers N] [--chapters N] [--pages N] [--think ms] [--ahead N] [--manga text] [--chapter-only] [--json]');
+    process.exit(o.help ? 0 : 2);
+  }
+  const cookie = await login(o);
+  let mangas = (await json(o, cookie, '/api/library')).data.mangas;
+  if (o.manga) mangas = mangas.filter(m => m.title.toLowerCase().includes(o.manga.toLowerCase()));
+  if (!mangas.length) throw new Error('no manga to read');
+  const out = { pages: [], api: [] }, t0 = Date.now();
+  await Promise.all(Array.from({ length: o.readers }, (_, i) => reader(o, cookie, mangas, out, i)));
+  const ok = out.pages.filter(p => p.status === 200), mb = ok.reduce((a, p) => a + p.bytes, 0) / 1048576;
+  const result = {
+    readers: o.readers, seconds: Math.round((Date.now() - t0) / 1000), pages: out.pages.length, errors: out.pages.length - ok.length,
+    MB: Math.round(mb), wait: sum(ok.map(p => p.wait)), ttfb: sum(ok.map(p => p.ttfb)), total: sum(ok.map(p => p.ms)),
+    waitedOver300: ok.filter(p => p.wait >= 300).length, waitedOver1s: ok.filter(p => p.wait >= 1000).length,
+    api: Object.fromEntries([...new Set(out.api.map(a => a.path))].map(p => [p, sum(out.api.filter(a => a.path === p).map(a => a.ms))])),
+    worst: ok.sort((a, b) => b.wait - a.wait).slice(0, 8).map(p => ({ wait: Math.round(p.wait), ttfb: Math.round(p.ttfb), ms: Math.round(p.ms), kb: Math.round(p.bytes / 1024), at: `${p.manga} ${p.chapter} p${p.page}` })),
+  };
+  if (o.json) return console.log(JSON.stringify(result, null, 2));
+  console.log(`${result.readers} readers, ${result.pages} pages (${result.MB} MB) in ${result.seconds} s, ${result.errors} errors`);
+  for (const k of ['wait', 'ttfb', 'total']) console.log(`  ${k.padEnd(6)} ms ${JSON.stringify(result[k])}`);
+  console.log(`  page waits >= 300 ms: ${result.waitedOver300}, >= 1 s: ${result.waitedOver1s}  (wait = time the reader looked at an empty page)`);
+  for (const [p, s] of Object.entries(result.api)) console.log(`  ${p} ${JSON.stringify(s)}`);
+  for (const w of result.worst) console.log(`    wait ${w.wait} ms (ttfb ${w.ttfb}, total ${w.ms}, ${w.kb} KB) ${w.at}`);
+}
+
+main().catch(e => { console.error(e.message); process.exit(1); });
diff --git a/tools/perf-report.js b/tools/perf-report.js
new file mode 100644
index 0000000..809e6a4
--- /dev/null
+++ b/tools/perf-report.js
@@ -0,0 +1,196 @@
+#!/usr/bin/env node
+'use strict';
+/*
+ * Summarise a performance trace (docs/performance.md) and say where slow page loads spend their time.
+ *
+ *   node tools/perf-report.js DATA/perf                       all trace files in the folder
+ *   node tools/perf-report.js trace-2026-10-29.jsonl --since 2026-10-29T18:00 --until …  --slow 500 --json
+ *
+ * Plain text by default (readable by people and agents); --json prints the numbers only.
+ */
+const fs = require('fs'), path = require('path');
+
+function parseArgs(argv) {
+  const opts = { files: [], since: null, until: null, slow: 500, json: false };
+  for (let i = 0; i < argv.length; i++) {
+    const a = argv[i];
+    if (a === '--since') opts.since = Date.parse(argv[++i]);
+    else if (a === '--until') opts.until = Date.parse(argv[++i]);
+    else if (a === '--slow') opts.slow = +argv[++i];
+    else if (a === '--json') opts.json = true;
+    else if (a === '-h' || a === '--help') opts.help = true;
+    else opts.files.push(a);
+  }
+  return opts;
+}
+
+function readEvents(inputs, { since, until }) {
+  const files = inputs.flatMap(p => fs.statSync(p).isDirectory()
+    ? fs.readdirSync(p).filter(n => /^trace-.*\.jsonl$/.test(n)).sort().map(n => path.join(p, n)) : [p]);
+  const events = [];
+  for (const file of files) for (const line of fs.readFileSync(file, 'utf8').split('\n')) {
+    if (!line) continue;
+    let e; try { e = JSON.parse(line); } catch { continue; }
+    if ((since && e.t < since) || (until && e.t > until)) continue;
+    events.push(e);
+  }
+  return events.sort((a, b) => a.t - b.t);
+}
+
+const pct = (values, p) => {
+  const v = values.filter(x => typeof x === 'number').sort((a, b) => a - b);
+  if (!v.length) return null;
+  return v[Math.min(v.length - 1, Math.floor((p / 100) * v.length))];
+};
+const stats = values => {
+  const v = values.filter(x => typeof x === 'number');
+  return v.length ? { n: v.length, p50: pct(v, 50), p90: pct(v, 90), p99: pct(v, 99), max: Math.max(...v) } : { n: 0 };
+};
+const isPageImage = e => e.k === 'req' && e.path.startsWith('/lib/') && e.path.split('/').length >= 6;
+
+// Where one served image spent its time on the server.
+function serverPhases(e) {
+  const stat = e.stat ?? 0, open = e.open != null ? e.open - stat : 0, firstRead = e.first != null && e.open != null ? e.first - e.open : 0;
+  const rest = Math.max(0, (e.disk ?? 0) - firstRead), net = e.netWait ?? 0;
+  const tail = e.readEnd != null ? Math.max(0, e.ms - e.readEnd) : 0;
+  return { stat, open, firstRead, readRest: rest, netWait: net, tail };
+}
+const dominant = phases => Object.entries(phases).sort((a, b) => b[1] - a[1])[0][0];
+
+function analyse(events, { slow }) {
+  const out = { window: events.length ? { from: new Date(events[0].t).toISOString(), to: new Date(events.at(-1).t).toISOString() } : null };
+  const reqs = events.filter(e => e.k === 'req');
+  const imgs = reqs.filter(isPageImage), served = imgs.filter(e => e.status === 200 && !e.aborted);
+  out.requests = { total: reqs.length, pageImages: imgs.length, served: served.length,
+    notModified: imgs.filter(e => e.status === 304).length, aborted: imgs.filter(e => e.aborted).length,
+    errors: imgs.filter(e => e.status >= 400).length, keepAliveReuse: imgs.length ? round(imgs.filter(e => e.conn > 1).length / imgs.length) : null };
+  out.serverImage = { total: stats(served.map(e => e.ms)), stat: stats(served.map(e => e.stat)),
+    firstRead: stats(served.map(e => serverPhases(e).firstRead)), disk: stats(served.map(e => e.disk)),
+    netWait: stats(served.map(e => e.netWait)), kbytes: stats(served.map(e => e.bytes / 1024)) };
+  const slowServed = served.filter(e => e.ms >= slow);
+  out.slowServer = { count: slowServed.length, share: served.length ? round(slowServed.length / served.length) : null, by: {} };
+  for (const e of slowServed) { const d = dominant(serverPhases(e)); out.slowServer.by[d] = (out.slowServer.by[d] || 0) + 1; }
+  out.slowServer.worst = slowServed.sort((a, b) => b.ms - a.ms).slice(0, 10)
+    .map(e => ({ at: new Date(e.t).toISOString(), ms: e.ms, ...serverPhases(e), path: e.path }));
+
+  const api = {};
+  for (const e of reqs.filter(e => e.path.startsWith('/api/'))) (api[e.path] ||= []).push(e.ms);
+  out.api = Object.fromEntries(Object.entries(api).map(([p, v]) => [p, stats(v)]).sort((a, b) => (b[1].p99 || 0) - (a[1].p99 || 0)).slice(0, 12));
+
+  // Browser view, matched to the server's record of the same file where possible.
+  const res = events.filter(e => e.k === 'client' && e.e === 'res'), byPath = new Map();
+  const decoded = p => { try { return decodeURIComponent(p); } catch { return p; } };
+  for (const e of imgs) { const key = decoded(e.path); (byPath.get(key) || byPath.set(key, []).get(key)).push(e); }
+  const matched = res.map(r => {
+    const cands = (byPath.get(r.url) || []).filter(s => s.t <= r.t + 1000 && s.t >= r.t - 120000);
+    const s = cands.at(-1);
+    return { ...r, serverMs: s ? s.ms : null };
+  });
+  const loaded = matched.filter(r => !r.cached);
+  out.browserImage = { total: stats(loaded.map(r => r.total)), queue: stats(loaded.map(r => r.queue)),
+    ttfb: stats(loaded.map(r => r.ttfb)), download: stats(loaded.map(r => r.download)),
+    connect: stats(loaded.map(r => r.connect)), dns: stats(loaded.map(r => r.dns)), fromCache: matched.length - loaded.length };
+  const slowBrowser = loaded.filter(r => r.total >= slow);
+  out.slowBrowser = { count: slowBrowser.length, share: loaded.length ? round(slowBrowser.length / loaded.length) : null, by: {} };
+  for (const r of slowBrowser) {
+    const parts = { server: r.serverMs ?? 0, network: Math.max(0, (r.ttfb ?? 0) + (r.download ?? 0) - (r.serverMs ?? 0)),
+      queue: r.queue ?? 0, connect: (r.connect ?? 0) + (r.dns ?? 0) };
+    const d = r.serverMs == null ? 'unmatched' : dominant(parts);
+    out.slowBrowser.by[d] = (out.slowBrowser.by[d] || 0) + 1;
+  }
+  const views = events.filter(e => e.k === 'client' && e.e === 'view'), stalls = events.filter(e => e.k === 'client' && e.e === 'stall');
+  out.pageTurns = { count: views.length, preloaded: views.filter(v => v.preloaded).length, wait: stats(views.map(v => v.wait)),
+    over300ms: views.filter(v => v.wait >= 300).length, over1s: views.filter(v => v.wait >= 1000).length };
+  out.flowStalls = { count: stalls.length, wait: stats(stalls.map(s => s.wait)), over1s: stalls.filter(s => s.wait >= 1000).length };
+  out.clients = [...new Set(events.filter(e => e.k === 'client' && e.e === 'hello').map(e => `${e.user}: ${e.ua}`))].slice(0, 10);
+
+  // Storage probe: cold reads of random pages; a long read after quiet time points at disks waking up.
+  const nas = events.filter(e => e.k === 'nas' && !e.error);
+  let lastIo = null;
+  const ioTimes = events.filter(e => isPageImage(e) || e.k === 'nas' || e.k === 'ahead').map(e => e.t);
+  out.nas = { probes: nas.length, errors: events.filter(e => e.k === 'nas' && e.error).length, first: stats(nas.map(e => e.firstMs)), total: stats(nas.map(e => e.ms)) };
+  out.nas.slow = nas.filter(e => e.firstMs >= 1000).map(e => {
+    let i = ioTimes.findIndex(t => t >= e.t); lastIo = i > 0 ? ioTimes[i - 1] : null;
+    return { at: new Date(e.t).toISOString(), firstMs: e.firstMs, quietMinBefore: lastIo ? round((e.t - e.ms - lastIo) / 60000) : null };
+  }).slice(0, 30);
+
+  const loops = events.filter(e => e.k === 'loop');
+  out.eventLoop = { samples: loops.length, lagP99: stats(loops.map(e => e.lagP99)), lagMax: stats(loops.map(e => e.lagMax)), cpu: stats(loops.map(e => e.cpu)), rssMB: stats(loops.map(e => e.rss)) };
+
+  const scans = events.filter(e => e.k === 'scan');
+  out.scans = { count: scans.length, ms: stats(scans.map(e => e.ms)) };
+  const during = e => scans.some(s => e.t >= s.t - s.ms && e.t - e.ms <= s.t);
+  out.scans.slowImagesDuringScan = slowServed.filter(during).length;
+  const span = events.length ? events.at(-1).t - events[0].t : 0;
+  out.scans.timeShare = span ? round(scans.reduce((a, s) => a + s.ms, 0) / span) : null;
+
+  const ahead = events.filter(e => e.k === 'ahead');
+  out.readAhead = { batches: ahead.length, replaced: ahead.filter(e => e.replaced).length, files: ahead.reduce((a, e) => a + e.done, 0),
+    MBps: stats(ahead.filter(e => e.ms > 0 && e.bytes > 0).map(e => round(e.bytes / 1048.576 / e.ms))) };
+  const scraper = events.filter(e => e.k === 'scraper');
+  out.scraper = { calls: scraper.length, errors: scraper.filter(e => e.error).length, ms: stats(scraper.map(e => e.ms)) };
+  out.verdict = verdict(out, slow);
+  return out;
+}
+
+function verdict(o, slow) {
+  const lines = [];
+  const top = by => Object.entries(by).sort((a, b) => b[1] - a[1]);
+  if (o.slowServer.count) {
+    const [[phase, n]] = top(o.slowServer.by);
+    const name = { stat: 'the file lookup (stat) on the storage', open: 'opening the file', firstRead: 'the first read from storage',
+      readRest: 'reading the rest of the file from storage', netWait: 'the client receiving data (network)', tail: 'flushing to the client' }[phase];
+    lines.push(`${o.slowServer.count} of ${o.serverImage.total.n} served pages took >= ${slow} ms on the server; ${n} of them mostly in ${name}.`);
+  } else if (o.serverImage.total.n) lines.push(`No served page took >= ${slow} ms on the server (p99 ${o.serverImage.total.p99} ms).`);
+  if (o.slowBrowser.count) {
+    const [[part, n]] = top(o.slowBrowser.by);
+    lines.push(`${o.slowBrowser.count} of ${o.browserImage.total.n} page loads took >= ${slow} ms in the browser; mostly ${part} (${n}).`);
+  }
+  if (o.pageTurns.count) lines.push(`Page turns: ${o.pageTurns.preloaded}/${o.pageTurns.count} preloaded, ${o.pageTurns.over300ms} waited >= 300 ms, ${o.pageTurns.over1s} >= 1 s (p90 ${o.pageTurns.wait.p90 ?? 0} ms).`);
+  if (o.flowStalls.count) lines.push(`Flow mode: ${o.flowStalls.count} pages were on screen before their image (p90 ${o.flowStalls.wait.p90} ms, ${o.flowStalls.over1s} >= 1 s).`);
+  if (o.nas.probes) {
+    lines.push(`Storage probe (cold page): first byte p50 ${o.nas.first.p50} ms, p99 ${o.nas.first.p99} ms, max ${o.nas.first.max} ms; ${o.nas.slow.length} probes >= 1 s.`);
+    const sleepy = o.nas.slow.filter(s => s.quietMinBefore >= 5).length;
+    if (sleepy) lines.push(`${sleepy} slow probes followed >= 5 min without library reads: disks waking from standby is likely.`);
+  }
+  if (o.eventLoop.samples && (o.eventLoop.lagMax.max || 0) >= 200) lines.push(`Event loop blocked up to ${o.eventLoop.lagMax.max} ms (reader CPU work holds requests back).`);
+  if (o.scans.slowImagesDuringScan) lines.push(`${o.scans.slowImagesDuringScan} slow pages overlapped a library scan (scans ran ${Math.round((o.scans.timeShare || 0) * 100)}% of the time).`);
+  return lines;
+}
+
+function print(o) {
+  const s = x => (x && x.n ? `n=${x.n} p50=${x.p50} p90=${x.p90} p99=${x.p99} max=${x.max}` : 'n=0');
+  const out = [];
+  out.push(`Trace ${o.window ? `${o.window.from} .. ${o.window.to}` : '(empty)'}`, '', 'Verdict:', ...o.verdict.map(l => '  - ' + l), '');
+  out.push('Requests', `  ${JSON.stringify(o.requests)}`);
+  out.push('Server, page images (ms)');
+  for (const k of ['total', 'stat', 'firstRead', 'disk', 'netWait', 'kbytes']) out.push(`  ${k.padEnd(10)} ${s(o.serverImage[k])}`);
+  out.push(`  slow by phase ${JSON.stringify(o.slowServer.by)}`);
+  for (const w of o.slowServer.worst) out.push(`    ${w.at} ${w.ms} ms stat=${w.stat} first=${w.firstRead} rest=${w.readRest} net=${w.netWait} ${w.path}`);
+  out.push('Browser, page images (ms)');
+  for (const k of ['total', 'queue', 'ttfb', 'download', 'connect', 'dns']) out.push(`  ${k.padEnd(10)} ${s(o.browserImage[k])}`);
+  out.push(`  from browser cache ${o.browserImage.fromCache}; slow by part ${JSON.stringify(o.slowBrowser.by)}`);
+  out.push(`Page turns  ${o.pageTurns.count} (${o.pageTurns.preloaded} preloaded) wait ${s(o.pageTurns.wait)}`);
+  out.push(`Flow stalls ${o.flowStalls.count} wait ${s(o.flowStalls.wait)}`);
+  out.push(`Storage probe first byte ${s(o.nas.first)}  total ${s(o.nas.total)}  errors ${o.nas.errors}`);
+  for (const p of o.nas.slow) out.push(`    ${p.at} first=${p.firstMs} ms after ${p.quietMinBefore ?? '?'} min quiet`);
+  out.push(`Event loop lagP99 ${s(o.eventLoop.lagP99)}  lagMax ${s(o.eventLoop.lagMax)}  cpu% ${s(o.eventLoop.cpu)}  rssMB ${s(o.eventLoop.rssMB)}`);
+  out.push(`Scans ${o.scans.count} ${s(o.scans.ms)}`);
+  out.push(`Read-ahead ${JSON.stringify(o.readAhead)}`);
+  out.push(`Scraper calls ${o.scraper.calls} errors ${o.scraper.errors} ${s(o.scraper.ms)}`);
+  out.push('API (ms, slowest p99 first)');
+  for (const [p, v] of Object.entries(o.api)) out.push(`  ${p.padEnd(28)} ${s(v)}`);
+  if (o.clients.length) out.push('Clients', ...o.clients.map(c => '  ' + c));
+  return out.join('\n');
+}
+
+const round = v => Math.round(v * 100) / 100;
+
+if (require.main === module) {
+  const opts = parseArgs(process.argv.slice(2));
+  if (opts.help || !opts.files.length) { console.log('usage: node tools/perf-report.js <trace.jsonl|folder>... [--since ISO] [--until ISO] [--slow ms] [--json]'); process.exit(opts.help ? 0 : 2); }
+  const result = analyse(readEvents(opts.files, opts), opts);
+  console.log(opts.json ? JSON.stringify(result, null, 2) : print(result));
+}
+
+module.exports = { analyse, readEvents, print };