Fixes#1258 — Perf dashboard (/#/perf) was slow because of three
frontend issues; backend APIs were never the problem.
## Findings
1. **`/api/health` fetched sequentially after `Promise.all`** in
`refresh()` — added a full RTT (~50-200ms) on every 5s tick on top of
the parallel batch.
2. **Endpoints table not actually sorted** despite the heading "sorted
by total time". JSON shape is `map[string]EndpointStatsResp` (no defined
order); frontend rendered map iteration order. Visible correctness bug
surfaced during investigation.
3. **`setInterval(refresh, 5000)` kept firing while tab was hidden**,
rebuilding the entire ~10-section `innerHTML` (cards + 3 tables) in the
background. On tab return the user saw a backlog thrash + felt the page
was "slow to render".
## Fix (`public/perf.js`)
- Move `/api/health` into the same `Promise.all` as the other 4
endpoints — saves one RTT per refresh.
- Sort `Object.entries(server.endpoints)` by `count * avgMs` DESC
client-side.
- Add `document.hidden` guard in the interval tick + `visibilitychange`
listener that refreshes once on return; `destroy()` removes the
listener.
## Tests
`test-perf-render-1258.js` (new):
- All 5 initial fetches issued in parallel (including `/api/health`)
- Refresh suppressed while `document.hidden`
- Endpoints table sorted by total time DESC, regardless of input map
order
RED commit first (`6b54f9e8`, 0/3 pass) → GREEN commit (`be81303b`, 3/3
pass). Existing `test-perf-go-runtime.js` (13/13) and
`test-perf-disk-io-1120.js` (15/15) still green.
## Investigation exemption
No Playwright timing test — sandbox can't run a real browser. Static
analysis + render-shape unit tests cover the three identified
bottlenecks. Documented per AGENTS "investigation surfaces" exemption.
## Measurement
Before: refresh = parallel batch (~max(server-side)) + sequential
`/api/health` (~50ms) + full innerHTML rebuild every 5s including hidden
tabs.
After: refresh = single parallel batch, runs only while visible.
Expected improvement on tab-return ≈ -1 RTT per refresh + zero
background work.
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Co-authored-by: corescope-bot <bot@corescope.local>
Kpa-clawbot