web-performance-audit · secondsky — Claude Code skill: Core Web Vitals audit

---
name: web-performance-audit
description: Web performance audits with Core Web Vitals, bottleneck identification, optimization recommendations. Use for page load times, performance reviews, UX optimization, or encountering LCP, FID, CLS issues, resource blocking, render delays.
license: MIT
---

# Web Performance Audit

Conduct comprehensive performance audits and implement optimizations.

## Core Web Vitals Targets

| Metric | Good | Needs Improvement | Poor |
|--------|------|-------------------|------|
| LCP (Largest Contentful Paint) | <2.5s | 2.5-4s | >4s |
| FID (First Input Delay) | <100ms | 100-300ms | >300ms |
| CLS (Cumulative Layout Shift) | <0.1 | 0.1-0.25 | >0.25 |
| INP (Interaction to Next Paint) | <200ms | 200-500ms | >500ms |

## Performance Measurement

```javascript
// Using web-vitals library
import { getCLS, getFID, getLCP, getTTFB } from 'web-vitals';

function sendToAnalytics({ name, value, id }) {
  console.log({ name, value, id });
}

getCLS(sendToAnalytics);
getFID(sendToAnalytics);
getLCP(sendToAnalytics);
getTTFB(sendToAnalytics);
```

## Lighthouse Automation

```javascript
const lighthouse = require('lighthouse');
const chromeLauncher = require('chrome-launcher');

async function runAudit(url) {
  const chrome = await chromeLauncher.launch({ chromeFlags: ['--headless'] });
  const result = await lighthouse(url, {
    port: chrome.port,
    onlyCategories: ['performance']
  });

  await chrome.kill();
  return result.lhr;
}
```

## Optimization Strategies

### Quick Wins (1-2 days)
- Enable gzip/brotli compression
- Minify CSS/JS
- Defer non-critical scripts
- Optimize images (WebP, lazy loading)

### Medium Effort (1-2 weeks)
- Implement code splitting
- Add service worker caching
- Preload critical resources
- Eliminate render-blocking resources

### Long-term (1-3 months)
- Architecture improvements
- CDN optimization
- Database query optimization

## Performance Budget

```json
{
  "timings": [
    { "metric": "first-contentful-paint", "budget": 1500 },
    { "metric": "largest-contentful-paint", "budget": 2500 }
  ],
  "resourceSizes": [
    { "resourceType": "script", "budget": 150 },
    { "resourceType": "image", "budget": 300 }
  ]
}
```

## Audit Checklist

- [ ] Measure baseline Core Web Vitals
- [ ] Run Lighthouse audit (mobile & desktop)
- [ ] Analyze real user metrics (RUM)
- [ ] Identify largest contentful element
- [ ] Check for layout shifts
- [ ] Review JavaScript bundle size
- [ ] Test on slow 3G connection
- [ ] Set performance budget
- [ ] Configure monitoring alerts