5chan/docs/agent-playbooks/low-spec-verification.md

Low-Spec Device Verification

Use this playbook when you need to know whether loading, navigation, or interaction speed is a real user problem or just an artifact of a fast development machine. It adds CPU + network throttling on top of the normal playwright-cli verification flow.

When to run

• A change touches navigation, data loading, list/feed rendering, images/media, or anything where "is this slow?" matters.
• You cannot tell whether jank or lag is real because the dev machine is high-spec.
• Before claiming a perf-sensitive change "feels fast".

This is an extra pass, not a replacement for the standard Chrome/Firefox/WebKit verification.

How it works

playwright-cli (@playwright/cli) has no throttle command, so throttling is applied through CDP via run-code. scripts/pw-throttle.sh wraps that call. The throttle is set on the page target and persists for the life of the -s=<session>, so apply it once, then run the usual goto / snapshot / screenshot commands on the same session.

Throttling is Chromium-only (CDP). It does not work on firefox or webkit sessions — run those engines unthrottled.

Profiles

Profile	CPU	Network (down / up / latency)	Models
mid	4x	~1.6 Mbps / 750 Kbps / 150 ms	mid-tier phone (Lighthouse mobile default)
low	6x	~400 Kbps / 400 Kbps / 400 ms	low-end phone / poor connection
cpu4	4x	full speed	isolate JS cost from network
cpu6	6x	full speed	isolate JS cost from network
off	1x	full speed	reset to normal

Usage

# open a Chromium session, throttle it to a mid-tier phone, then verify as usual
playwright-cli -s=lowspec open https://5chan.localhost --browser=chrome
./scripts/pw-throttle.sh lowspec mid
playwright-cli -s=lowspec snapshot
playwright-cli -s=lowspec screenshot --filename=lowspec-mid.png

# tighten to a low-end device
./scripts/pw-throttle.sh lowspec low

# isolate JS cost (CPU throttle, full-speed network)
./scripts/pw-throttle.sh lowspec cpu4

# reset and finish
./scripts/pw-throttle.sh lowspec off
playwright-cli -s=lowspec close

Measuring, not guessing

Throttled numbers are approximations (CDP throttling, not a real device), but the relative difference is the signal. For the initial page load:

playwright-cli -s=lowspec eval "() => Math.round(performance.getEntriesByType('navigation')[0].duration)"

5chan is a React Router SPA, so in-app navigation does not create a new navigation entry. Time those manually: read performance.now() before triggering the route change, then again once the target content appears in the snapshot.

Caveats

• Chromium-only. Skip on Firefox/WebKit sessions; keep those checks unthrottled.
• The low latency is intentionally aggressive; if requests time out, fall back to mid.
• For render/rerender hotspots after a slow result, use the profile-browsing skill (it drives playwright-cli + react-scan) on the already-throttled session.