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Trace cold request waterfalls and isolate the layout discovery regression

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Jonathan Sykes
2026-10-08 08:45:00 +08:00
parent af6aabc3b7
commit 799b1e7ed6
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# Phase 6 investigation
All comparisons use the current instrumented harness with matching frontend and
server from detached worktrees beneath `perf/.tmp`. No production requests.
Command template (three fresh contexts):
```
node perf/baseline.mjs --runs 3 --browser webkit --profile lte --scenario cold --source-root perf/.tmp/phase6-<variant> --out perf/results/phase6-<variant>.json
```
| Source | Commit | LTE FCP median ms | Wire bytes | app.js positive responses/sample |
|---|---|---:|---:|---:|
| Phase 2 | 4950d3e | 1044 | 1191063 | 2 |
| Immediately before lazy layouts | d98d444 | 1032 | 1191686 | 2 |
| First lazy-layout cut | 5d6a154 | 1615 | 1106796 | 2 |
| Phase 3 tip | cd4a25a | 1646 | 1108191 | 2 |
| Phase 5 tip (same product as Phase 4) | 21c1273 | 1736 | 1097007 | 2 |
| Phase 5 + worker force-cache | scratch | 1745 | 1097006 | 2 |
| Phase 5 + parser-owned classic CSS | scratch | 898 | 1097051 | 2 |
| Phase 5 + classic preload | scratch | 909 | 1097032 | 2 |
The adjacent-commit measurements isolate `5d6a154`, not the Phase 4 extraction.
All three FCP samples, complete proxy waterfalls, and page Resource Timing are
stored in each result. The first sample often includes startup compression cost;
it is retained in every median, not discarded.
Phase 3 sample 2: common styles finish around 880 ms. The parser-written classic
stylesheet is discovered at 416 ms but reaches the proxy only at 1434 ms and
finishes at 1607 ms. FCP is 1646 ms. Phase 5 sample 2: classic reaches the proxy
at 1540 ms, finishes at 1711 ms, FCP is 1736 ms. Speculatively discovered body
scripts already occupy the request queue when the selected stylesheet arrives.
Phase 2 discovers its declarative styles together around 231 ms, FCP 1044 ms.
The first service-worker request is at 2458 ms in Phase 3 and 2337 ms in Phase 5,
AFTER paint. Worker install competition does not explain this FCP regression.
Font preload URLs match the font CSS; late layout CSS is the final rendering
barrier. Making classic CSS declarative again removes the delay without changing
CSS bytes or cascade order. This scratch variant unconditionally applies classic
and is diagnostic only; the shipping fix must retain selected-layout behavior.
The modern synchronizer already uses default fetch caching. Changing it to
force-cache has no effect: every WebKit sample still transfers app.js twice.
The page and worker cannot reuse these HTTP-cache responses in this harness.
A same-build page handoff can reuse the page HTTP cache instead. Worker validation
must check both the X-Asset-Hash header and SHA-256 of the decoded bytes. It must
strip compressed transport headers before constructing the cached Response.
Updates and legacy migration must continue through their existing worker path.
The shipping CSS fix is a preload, not an unconditional stylesheet. Its three-run
FCP is 909 ms; the first classic request reaches the proxy at 250 ms and completes
at 423 ms. Lazy still applies only the saved layout, in the original cascade order.
Other selected layouts may speculatively download the small classic stylesheet;
it never applies or blocks their rendering. No inline script or CSP change.