Web Performance Optimization

# Web Performance Optimization

## Overview

Help developers optimize website and web application performance to improve user experience, SEO rankings, and conversion rates. This skill provides systematic approaches to measure, analyze, and improve loading speed, runtime performance, and Core Web Vitals metrics.

## When to Use This Skill

- Use when website or app is loading slowly
- Use when optimizing for Core Web Vitals (LCP, FID, CLS)
- Use when reducing JavaScript bundle size
- Use when improving Time to Interactive (TTI)
- Use when optimizing images and assets
- Use when implementing caching strategies
- Use when debugging performance bottlenecks
- Use when preparing for performance audits

## How It Works

### Step 1: Measure Current Performance

I'll help you establish baseline metrics:
- Run Lighthouse audits
- Measure Core Web Vitals (LCP, FID, CLS)
- Check bundle sizes
- Analyze network waterfall
- Identify performance bottlenecks

### Step 2: Identify Issues

Analyze performance problems:
- Large JavaScript bundles
- Unoptimized images
- Render-blocking resources
- Slow server response times
- Missing caching headers
- Layout shifts
- Long tasks blocking main thread

### Step 3: Prioritize Optimizations

Focus on high-impact improvements:
- Critical rendering path optimization
- Code splitting and lazy loading
- Image optimization
- Caching strategies
- Third-party script optimization

### Step 4: Implement Optimizations

Apply performance improvements:
- Optimize assets (images, fonts, CSS, JS)
- Implement code splitting
- Add caching headers
- Lazy load non-critical resources
- Optimize critical rendering path

### Step 5: Verify Improvements

Measure impact of changes:
- Re-run Lighthouse audits
- Compare before/after metrics
- Monitor real user metrics (RUM)
- Test on different devices and networks

## Examples

### Example 1: Optimizing Core Web Vitals

```markdown
## Performance Audit Results

### Current Metrics (Before Optimization)
- **LCP (Largest Contentful Paint):** 4.2s ❌ (should be < 2.5s)
- **FID (First Input Delay):** 180ms ❌ (should be < 100ms)
- **CLS (Cumulative Layout Shift):** 0.25 ❌ (should be < 0.1)
- **Lighthouse Score:** 62/100

### Issues Identified

1. **LCP Issue:** Hero image (2.5MB) loads slowly
2. **FID Issue:** Large JavaScript bundle (850KB) blocks main thread
3. **CLS Issue:** Images without dimensions cause layout shifts

### Optimization Plan

#### Fix LCP (Largest Contentful Paint)

**Problem:** Hero image is 2.5MB and loads slowly

**Solutions:**
\`\`\`html
<!-- Before: Unoptimized image -->
<img src="/hero.jpg" alt="Hero">

<!-- After: Optimized with modern formats -->
<picture>
  <source srcset="/hero.avif" type="image/avif">
  <source srcset="/hero.webp" type="image/webp">
  <img 
    src="/hero.jpg" 
    alt="Hero"
    width="1200" 
    height="600"
    loading="eager"
    fetchpriority="high"
  >
</picture>
\`\`\`

**Additional optimizations:**
- Compress image to < 200KB
- Use CDN for faster delivery
- Preload hero image: `<link rel="preload" as="image" href="/hero.avif">`

#### Fix FID (First Input Delay)

**Problem:** 850KB JavaScript bundle blocks main thread

**Solutions:**

1. **Code Splitting:**
\`\`\`javascript
// Before: Everything in one bundle
import { HeavyComponent } from './HeavyComponent';
import { Analytics } from './analytics';
import { ChatWidget } from './chat';

// After: Lazy load non-critical code
const HeavyComponent = lazy(() => import('./HeavyComponent'));
const ChatWidget = lazy(() => import('./chat'));

// Load analytics after page interactive
if (typeof window !== 'undefined') {
  window.addEventListener('load', () => {
    import('./analytics').then(({ Analytics }) => {
      Analytic