diff --git a/.env.example b/.env.example
index 3d8943d..98a9346 100644
--- a/.env.example
+++ b/.env.example
@@ -20,6 +20,10 @@ READING_TIMEZONE=UTC
 READING_QUIZ_UNLOCK_MINUTES=60
 # Optional deployment-managed JSON file under DATA_PATH, e.g. /data/reading-quiz.json.
 # READING_QUIZ_CATALOG_PATH=/data/reading-quiz.json
+# Read an opened chapter into the OS cache ahead of the browser (1 = on, the default); see docs/performance.md.
+# READ_AHEAD=1
+# Performance trace from the start (also switchable at runtime: touch DATA_PATH/perf/trace.on).
+# PERF_TRACE=0
 PROXY_NETWORK=mangacircus-edge
 # Extra Compose files, ':'-separated (docs/deployment.md), e.g. HTTPS with the bundled Caddy:
 # COMPOSE_FILE=docker-compose.yml:deploy/compose.https.yml
diff --git a/docker-compose.yml b/docker-compose.yml
index ffe7d02..d0afe1f 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -61,6 +61,8 @@ services:
       DISCOVERY_HARVEST_AT: ${DISCOVERY_HARVEST_AT:-03:30}
       DISCOVERY_ANILIST_URL: ${DISCOVERY_ANILIST_URL:-}
       GREATEST_COVER_WARM: ${GREATEST_COVER_WARM:-1}
+      READ_AHEAD: ${READ_AHEAD:-1}
+      PERF_TRACE: ${PERF_TRACE:-0}
     networks: [backend, egress]
     healthcheck:
       test: [CMD, node, tools/healthcheck.js, reader]
diff --git a/perf/read-ahead.js b/perf/read-ahead.js
index 6caa2e7..4833cf9 100644
--- a/perf/read-ahead.js
+++ b/perf/read-ahead.js
@@ -5,49 +5,52 @@
  * 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.
+ * One queue per reader (`key`, the user): opening another chapter replaces that reader's queue only, and
+ * the queues take turns so two people reading at once both get their pages ahead.
  */
 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 = [];
+    this.batches = new Map(); this.turn = []; this.active = 0; this.recent = new Map(); 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);
+  warm(files, tag, key = '') {
+    const old = this.batches.get(key);
+    if (old) this.finish(key, old, true);
+    const queue = files.filter(f => !this.recent.has(f));
+    const batch = { tag, queue, files: queue.length, skipped: files.length - queue.length, bytes: 0, done: 0, active: 0, t0: performance.now() };
+    this.batches.set(key, batch);
+    if (!this.turn.includes(key)) this.turn.unshift(key); // a fresh request goes next
+    if (!queue.length) this.finish(key, batch, 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;
+  // Runs after the reader's current batch (e.g. the next chapter) unless a newer warm() replaces it first.
+  then(files, tag, key = '') {
+    const batch = this.batches.get(key);
+    if (!batch) this.warm(files, tag, key); else batch.next = () => this.warm(files, tag, key);
   }
 
   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);
+    while (this.active < this.concurrency) {
+      const key = this.turn.find(k => this.batches.get(k)?.queue.length);
+      if (key === undefined) return;
+      this.turn.push(...this.turn.splice(this.turn.indexOf(key), 1)); // round robin between readers
+      const batch = this.batches.get(key), file = batch.queue.shift();
+      this.active++; batch.active++;
+      this.readFile(file).then(bytes => { this.note(file); batch.bytes += bytes; batch.done++; }, () => {}).finally(() => {
+        this.active--; batch.active--;
+        if (this.batches.get(key) === batch && !batch.queue.length && !batch.active) this.finish(key, batch, false);
         this.pump();
       });
     }
   }
-  finishBatch(replaced) {
-    const b = this.batch; this.batch = null;
-    if (!b) return;
+  finish(key, b, replaced) {
+    if (this.batches.get(key) === b) this.batches.delete(key);
+    if (!this.batches.has(key)) this.turn = this.turn.filter(k => k !== key);
+    b.queue = [];
     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();
diff --git a/server.js b/server.js
index bce53ee..55ad355 100644
--- a/server.js
+++ b/server.js
@@ -448,14 +448,14 @@ function continueOf(m, prog, s) {
   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) {
+function warmChapter(u, 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}`);
+  readAhead.warm(pages, `${m.id}/${ch.id}`, u.name);
   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}`);
+    readAhead.then(nv.pages.map(p => path.join(LIB, nv.rel, p.f)), `${m.id}/${nextCh.id}`, u.name);
   }
 }
 function neighbour(m, ch, step, s) {
@@ -768,7 +768,7 @@ 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);
+    if (readAhead) warmChapter(u, m, ch, v, prog[ch.id], s);
     return {
       perf: tracer.on,
       manga: { id: m.id, title: m.title },
diff --git a/test/perf.test.js b/test/perf.test.js
index 0dd46ef..c777738 100644
--- a/test/perf.test.js
+++ b/test/perf.test.js
@@ -76,6 +76,19 @@ test('read-ahead reads files in order and a newer chapter replaces the queued on
   assert.deepEqual(batches.map(x => [x.tag, x.files, x.skipped]), [['C', 0, 2]]);
 });
 
+test('read-ahead keeps one queue per reader and serves them in turn', async () => {
+  const dir = tmp(), mk = n => { const f = path.join(dir, n); fs.writeFileSync(f, n); return f; };
+  const order = [], batches = [];
+  const ahead = new ReadAhead({ concurrency: 1, onBatch: b => batches.push(b) });
+  const read = ahead.readFile.bind(ahead);
+  ahead.readFile = f => { order.push(path.basename(f)); return read(f); };
+  ahead.warm([mk('a1'), mk('a2')], 'A', 'ann');
+  ahead.warm([mk('b1'), mk('b2')], 'B', 'bob');
+  await new Promise(r => { const check = () => (batches.length === 2 ? r() : setTimeout(check, 5)); check(); });
+  assert.deepEqual(order, ['a1', 'b1', 'a2', 'b2']);
+  assert.ok(batches.every(b => !b.replaced && b.done === 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');
diff --git a/tools/perf-report.js b/tools/perf-report.js
index 809e6a4..32a118a 100644
--- a/tools/perf-report.js
+++ b/tools/perf-report.js
@@ -53,7 +53,8 @@ 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 r = v => Math.round(v * 10) / 10;
+  return { stat: r(stat), open: r(open), firstRead: r(firstRead), readRest: r(rest), netWait: r(net), tail: r(tail) };
 }
 const dominant = phases => Object.entries(phases).sort((a, b) => b[1] - a[1])[0][0];
 
@@ -106,12 +107,12 @@ function analyse(events, { slow }) {
 
   // 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);
+  // Library reads that finished before a probe started (a probe's own read does not count).
+  const ioEnds = 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 };
+    const start = e.t - e.ms, before = ioEnds.filter(t => t < start).at(-1);
+    return { at: new Date(e.t).toISOString(), firstMs: e.firstMs, quietMinBefore: before ? round((start - before) / 60000) : null };
   }).slice(0, 30);
 
   const loops = events.filter(e => e.k === 'loop');