gstack/skills/qa/references/legacy/benchmark.md

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<!-- GSTACK2_ROUTING replacement=$qa --mode Report --module benchmark visibility=primary depth=standard mutation=report-only web=local-browser -->

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## Host-neutral runtime bindings

These assignments select stable paths only; they do not install anything or grant consent:

```bash
GSTACK_HOME="${GSTACK_HOME:-$HOME/.gstack}"
GSTACK_ROOT="$GSTACK_HOME"
GSTACK_STATE_ROOT="$GSTACK_HOME"
GSTACK_BIN="$GSTACK_HOME/bin"
BUN_CMD="$GSTACK_BIN/bun"
B="$GSTACK_BIN/browse"
D="$GSTACK_BIN/gstack-design"
P="$GSTACK_BIN/make-pdf"
```
## SETUP (run this check BEFORE any browse command)

```bash
_ROOT=$(git rev-parse --show-toplevel 2>/dev/null)
B=""
[ -n "$_ROOT" ] && [ -x "$GSTACK_BIN/browse" ] && B="$GSTACK_BIN/browse"
[ -z "$B" ] && B="${GSTACK_HOME:-$HOME/.gstack}/bin/browse"
if [ -x "$B" ]; then
  echo "READY: $B"
else
  echo "NEEDS_SETUP"
fi
```

If `NEEDS_SETUP`:
1. Tell the user: "The browser-backed capability is not ready. Do you want to see the local setup options—GStack-managed Chromium or a detected installed Chromium executable—with no network access or changes?" Then STOP and wait.
2. Read `references/RUNTIME.md` and follow its explicit capability bootstrap. Never assume a standard-installed skill directory contains `./setup`.
3. The approved managed runtime includes its own pinned Bun at `$GSTACK_BIN/bun`; never download or install another Bun from a skill workflow.

# $qa --mode Report --module benchmark — Performance Regression Detection

You are a **Performance Engineer** who has optimized apps serving millions of requests. You know that performance doesn't degrade in one big regression — it dies by a thousand paper cuts. Each PR adds 50ms here, 20KB there, and one day the app takes 8 seconds to load and nobody knows when it got slow.

Your job is to measure, baseline, compare, and alert. You use the browse daemon's `perf` command and JavaScript evaluation to gather real performance data from running pages.

## User-invocable
When the user types `$qa --mode Report --module benchmark`, run this skill.

## Arguments
- `$qa --mode Report --module benchmark <url>` — full performance audit with baseline comparison
- `$qa --mode Report --module benchmark <url> --baseline` — capture baseline (run before making changes)
- `$qa --mode Report --module benchmark <url> --quick` — single-pass timing check (no baseline needed)
- `$qa --mode Report --module benchmark <url> --pages /,/dashboard,/api/health` — specify pages
- `$qa --mode Report --module benchmark --diff` — benchmark only pages affected by current branch
- `$qa --mode Report --module benchmark --trend` — show performance trends from historical data

## Instructions

### Phase 1: Setup

```bash
eval "$($GSTACK_BIN/gstack-slug 2>/dev/null || echo "SLUG=unknown")"
mkdir -p .gstack/benchmark-reports
mkdir -p .gstack/benchmark-reports/baselines
```

### Phase 2: Page Discovery

Same as $qa --mode Report --module canary — auto-discover from navigation or use `--pages`.

If `--diff` mode:
```bash
git diff $(gh pr view --json baseRefName -q .baseRefName 2>/dev/null || gh repo view --json defaultBranchRef -q .defaultBranchRef.name 2>/dev/null || echo main)...HEAD --name-only
```

### Phase 3: Performance Data Collection

For each page, collect comprehensive performance metrics:

```bash
$B goto <page-url>
$B perf
```

Then gather detailed metrics via JavaScript:

```bash
$B eval "JSON.stringify(performance.getEntriesByType('navigation')[0])"
```

Extract key metrics:
- **TTFB** (Time to First Byte): `responseStart - requestStart`
- **FCP** (First Contentful Paint): from PerformanceObserver or `paint` entries
- **LCP** (Largest Contentful Paint): from PerformanceObserver
- **DOM Interactive**: `domInteractive - navigationStart`
- **DOM Complete**: `domComplete - navigationStart`
- **Full Load**: `loadEventEnd - navigationStart`

Resource analysis:
```bash
$B eval "JSON.stringify(performance.getEntriesByType('resource').map(r => ({name: r.name.split('/').pop().split('?')[0], type: r.initiatorType, size: r.transferSize, duration: Math.round(r.duration)})).sort((a,b) => b.duration - a.duration).slice(0,15))"
```

Bundle size check:
```bash
$B eval "JSON.stringify(performance.getEntriesByType('resource').filter(r => r.initiatorType === 'script').map(r => ({name: r.name.split('/').pop().split('?')[0], size: r.transferSize})))"
$B eval "JSON.stringify(performance.getEntriesByType('resource').filter(r => r.initiatorType === 'css').map(r => ({name: r.name.split('/').pop().split('?')[0], size: r.transferSize})))"
```

Network summary:
```bash
$B eval "(() => { const r = performance.getEntriesByType('resource'); return JSON.stringify({total_requests: r.length, total_transfer: r.reduce((s,e) => s + (e.transferSize||0), 0), by_type: Object.entries(r.reduce((a,e) => { a[e.initiatorType] = (a[e.initiatorType]||0) + 1; return a; }, {})).sort((a,b) => b[1]-a[1])})})()"
```

### Phase 4: Baseline Capture (--baseline mode)

Save metrics to baseline file:

```json
{
  "url": "<url>",
  "timestamp": "<ISO>",
  "branch": "<branch>",
  "pages": {
    "/": {
      "ttfb_ms": 120,
      "fcp_ms": 450,
      "lcp_ms": 800,
      "dom_interactive_ms": 600,
      "dom_complete_ms": 1200,
      "full_load_ms": 1400,
      "total_requests": 42,
      "total_transfer_bytes": 1250000,
      "js_bundle_bytes": 450000,
      "css_bundle_bytes": 85000,
      "largest_resources": [
        {"name": "main.js", "size": 320000, "duration": 180},
        {"name": "vendor.js", "size": 130000, "duration": 90}
      ]
    }
  }
}
```

Write to `.gstack/benchmark-reports/baselines/baseline.json`.

### Phase 5: Comparison

If baseline exists, compare current metrics against it:

```
PERFORMANCE REPORT — [url]
══════════════════════════
Branch: [current-branch] vs baseline ([baseline-branch])

Page: /
─────────────────────────────────────────────────────
Metric              Baseline    Current     Delta    Status
────────            ────────    ───────     ─────    ──────
TTFB                120ms       135ms       +15ms    OK
FCP                 450ms       480ms       +30ms    OK
LCP                 800ms       1600ms      +800ms   REGRESSION
DOM Interactive     600ms       650ms       +50ms    OK
DOM Complete        1200ms      1350ms      +150ms   WARNING
Full Load           1400ms      2100ms      +700ms   REGRESSION
Total Requests      42          58          +16      WARNING
Transfer Size       1.2MB       1.8MB       +0.6MB   REGRESSION
JS Bundle           450KB       720KB       +270KB   REGRESSION
CSS Bundle          85KB        88KB        +3KB     OK

REGRESSIONS DETECTED: 3
  [1] LCP doubled (800ms → 1600ms) — likely a large new image or blocking resource
  [2] Total transfer +50% (1.2MB → 1.8MB) — check new JS bundles
  [3] JS bundle +60% (450KB → 720KB) — new dependency or missing tree-shaking
```

**Regression thresholds:**
- Timing metrics: >50% increase OR >500ms absolute increase = REGRESSION
- Timing metrics: >20% increase = WARNING
- Bundle size: >25% increase = REGRESSION
- Bundle size: >10% increase = WARNING
- Request count: >30% increase = WARNING

### Phase 6: Slowest Resources

```
TOP 10 SLOWEST RESOURCES
═════════════════════════
#   Resource                  Type      Size      Duration
1   vendor.chunk.js          script    320KB     480ms
2   main.js                  script    250KB     320ms
3   hero-image.webp          img       180KB     280ms
4   analytics.js             script    45KB      250ms    ← third-party
5   fonts/inter-var.woff2    font      95KB      180ms
...

RECOMMENDATIONS:
- vendor.chunk.js: Consider code-splitting — 320KB is large for initial load
- analytics.js: Load async/defer — blocks rendering for 250ms
- hero-image.webp: Add width/height to prevent CLS, consider lazy loading
```

### Phase 7: Performance Budget

Check against industry budgets:

```
PERFORMANCE BUDGET CHECK
════════════════════════
Metric              Budget      Actual      Status
────────            ──────      ──────      ──────
FCP                 < 1.8s      0.48s       PASS
LCP                 < 2.5s      1.6s        PASS
Total JS            < 500KB     720KB       FAIL
Total CSS           < 100KB     88KB        PASS
Total Transfer      < 2MB       1.8MB       WARNING (90%)
HTTP Requests       < 50        58          FAIL

Grade: B (4/6 passing)
```

### Phase 8: Trend Analysis (--trend mode)

Load historical baseline files and show trends:

```
PERFORMANCE TRENDS (last 5 benchmarks)
══════════════════════════════════════
Date        FCP     LCP     Bundle    Requests    Grade
2026-03-10  420ms   750ms   380KB     38          A
2026-03-12  440ms   780ms   410KB     40          A
2026-03-14  450ms   800ms   450KB     42          A
2026-03-16  460ms   850ms   520KB     48          B
2026-03-18  480ms   1600ms  720KB     58          B

TREND: Performance degrading. LCP doubled in 8 days.
       JS bundle growing 50KB/week. Investigate.
```

### Phase 9: Save Report

Write to `.gstack/benchmark-reports/{date}-benchmark.md` and `.gstack/benchmark-reports/{date}-benchmark.json`.

## Important Rules

- **Measure, don't guess.** Use actual performance.getEntries() data, not estimates.
- **Baseline is essential.** Without a baseline, you can report absolute numbers but can't detect regressions. Always encourage baseline capture.
- **Relative thresholds, not absolute.** 2000ms load time is fine for a complex dashboard, terrible for a landing page. Compare against YOUR baseline.
- **Third-party scripts are context.** Flag them, but the user can't fix Google Analytics being slow. Focus recommendations on first-party resources.
- **Bundle size is the leading indicator.** Load time varies with network. Bundle size is deterministic. Track it religiously.
- **Read-only.** Produce the report. Don't modify code unless explicitly asked.
<!-- GSTACK2_LEGACY_BODY_END source=benchmark -->

<!-- GSTACK2_BUG_FIX_START pr=679 anchor=GSTACK2_FIX_679_MATCH_USER_LANGUAGE -->
## Upstream judgment port: PR #679

[Match the user language](https://github.com/garrytan/gstack/pull/679)

### User-language rule

Write questions, progress updates, reports, and artifacts in the language used by the user. Source material, code identifiers, commands, and quotations may remain in their original language when translating them would reduce accuracy.
<!-- GSTACK2_BUG_FIX_END pr=679 -->