benchmark — Claude Code skill: measure project performance

---
name: benchmark
description: Measure performance baselines and detect regressions across browser Core Web Vitals (LCP, INP, CLS, page weight), API endpoint latency percentiles, and build/test feedback times, with before/after comparison stored in git-tracked .ecc/benchmarks JSON. Use when checking page speed, responding to 'it feels slow' reports, verifying launch performance targets, or comparing stack alternatives.
license: MIT
metadata:
  origin: ECC
---

# Benchmark — Performance Baseline & Regression Detection

## When to Use

- Before and after a PR to measure performance impact
- Setting up performance baselines for a project
- When users report "it feels slow"
- Before a launch — ensure you meet performance targets
- Comparing your stack against alternatives

## How It Works

### Mode 1: Page Performance

Measures real browser metrics via browser MCP:

```
1. Navigate to each target URL
2. Measure Core Web Vitals:
   - LCP (Largest Contentful Paint) — target < 2.5s
   - CLS (Cumulative Layout Shift) — target < 0.1
   - INP (Interaction to Next Paint) — target < 200ms
   - FCP (First Contentful Paint) — target < 1.8s
   - TTFB (Time to First Byte) — target < 800ms
3. Measure resource sizes:
   - Total page weight (target < 1MB)
   - JS bundle size (target < 200KB gzipped)
   - CSS size
   - Image weight
   - Third-party script weight
4. Count network requests
5. Check for render-blocking resources
```

### Mode 2: API Performance

Benchmarks API endpoints:

```
1. Hit each endpoint 100 times
2. Measure: p50, p95, p99 latency
3. Track: response size, status codes
4. Test under load: 10 concurrent requests
5. Compare against SLA targets
```

### Mode 3: Build Performance

Measures development feedback loop:

```
1. Cold build time
2. Hot reload time (HMR)
3. Test suite duration
4. TypeScript check time
5. Lint time
6. Docker build time
```

### Mode 4: Before/After Comparison

Run before and after a change to measure impact:

```
/benchmark baseline    # saves current metrics
# ... make changes ...
/benchmark compare     # compares against baseline
```

Output:
```
| Metric | Before | After | Delta | Verdict |
|--------|--------|-------|-------|---------|
| LCP | 1.2s | 1.4s | +200ms | WARNING: WARN |
| Bundle | 180KB | 175KB | -5KB | ✓ BETTER |
| Build | 12s | 14s | +2s | WARNING: WARN |
```

## Output

Stores baselines in `.ecc/benchmarks/` as JSON. Git-tracked so the team shares baselines.

## Integration

- CI: run `/benchmark compare` on every PR
- Pair with `/canary-watch` for post-deploy monitoring
- Pair with `/browser-qa` for full pre-ship checklist