Document the measured first-scan speed on the server (BL-0043)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/import.md

@@ -161,6 +161,22 @@ Uncached chapter folders and their images are probed **`SCAN_CONCURRENCY` at a t
 | JPEG | first SOFn marker (the scan skips APPn/EXIF/ICC segments). If it isn't within 32 KB, then 256 KB, the whole file is read once |
 | AVIF / HEIF | first `ispe` box |
 
+### 3.1 Measured first-scan speed
+
+the server (Debian VM, 4 vCPU) reading a Synology over NFS 4.0, page cache dropped before each run, production
+running alongside. Each variant scanned a different manga of ~5,000 pages (the NAS caches what it has just read).
+
+| Variant | ms per image (round 1 / round 2) |
+|---|---|
+| sequential, 256 KB (before) | 7.4 / 9.7 |
+| 32 KB first read, `SCAN_CONCURRENCY=1` | 3.6 / 6.0 |
+| 32 KB first read, `SCAN_CONCURRENCY=8` (default) | 2.9 / 2.1 |
+| 32 KB first read, `SCAN_CONCURRENCY=16` | 2.4 / 3.3 |
+
+Whole library without cache (56 manga, 10,766 chapters, 299,467 pages) with the default: **825 s (13.8 min)**,
+against an estimated 37–48 min before (about 4 h while the NAS was busy with a large copy). The page lists and
+sizes of all 10,775 cached chapter folders were identical to those of the previous code.
+
 - If the size can't be read, it is stored as `0×0`. The reader then assumes 1000×1414 (2000×1414 for `double`) and corrects the layout when the image arrives.
 - EXIF orientation is **not** applied. Use `rot…` tags instead.