---
name: performance
description: Optimize web performance for faster loading and better user experience. Use when asked to "speed up my site", "optimize performance", "reduce load time", "fix slow loading", "improve page speed", or "performance audit".
license: MIT
metadata:
  author: web-quality-skills
  version: "1.0"
---

# Performance optimization

Deep performance optimization based on Lighthouse performance audits. Focuses on loading speed, runtime efficiency, and resource optimization.

## How it works

1. Identify performance bottlenecks in code and assets
2. Prioritize by impact on Core Web Vitals
3. Provide specific optimizations with code examples
4. Measure improvement with before/after metrics

## Performance budget

| Resource | Budget | Rationale |
|----------|--------|-----------|
| Total page weight | < 1.5 MB | 3G loads in ~4s |
| JavaScript (compressed) | < 300 KB | Parsing + execution time |
| CSS (compressed) | < 100 KB | Render blocking |
| Images (above-fold) | < 500 KB | LCP impact |
| Fonts | < 100 KB | FOIT/FOUT prevention |
| Third-party | < 200 KB | Uncontrolled latency |

## Critical rendering path

### Server response
* **TTFB < 800ms.** Time to First Byte should be fast. Use CDN, caching, and efficient backends.
* **Enable compression.** Gzip or Brotli for text assets. Brotli preferred (15-20% smaller).
* **HTTP/2 or HTTP/3.** Multiplexing reduces connection overhead.
* **Edge caching.** Cache HTML at CDN edge when possible.
* **Send Early Hints (HTTP 103) for slow origins.** When the origin needs hundreds of milliseconds to assemble the final response, return a `103 Early Hints` with `Link: </hero.webp>; rel=preload; as=image` (and similar for critical CSS/fonts) so the browser starts fetching before the `200 OK` lands. Cloudflare reports [20–30% LCP improvements](https://blog.cloudflare.com/early-hints-performance/) on image-heavy pages. Requires HTTP/2+ and is supported by Chromium-based browsers; other browsers ignore the 103 and fall through to the 200 — safe to enable. CDNs (Cloudflare, Fastly, Akamai) can synthesize 103s automatically from prior responses; on your own origin, emit them from the same handler that issues the 200.

### Resource loading

**Preconnect to required origins:**
```html
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com" crossorigin>
```

**Preload critical resources:**
```html
<!-- LCP image -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">

<!-- Critical font -->
<link rel="preload" href="/font.woff2" as="font" type="font/woff2" crossorigin>
```

**Prerender likely-next navigations** with the [Speculation Rules API](https://developer.chrome.com/docs/web-platform/prerender-pages):
```html
<script type="speculationrules">
{
  "prerender": [{
    "where": { "href_matches": "/*" },
    "eagerness": "moderate"
  }]
}
</script>
```
`moderate` triggers after a ~200ms hover — usually intent-correlated, rarely wasted. See [core-web-vitals → LCP](../core-web-vitals/SKILL.md#lcp-largest-contentful-paint) for the full discussion of eagerness tradeoffs and the `prerenderingchange` gating you'll need for analytics.

**Defer non-critical CSS:**
```html
<!-- Critical CSS inlined -->
<style>/* Above-fold styles */</style>

<!-- Non-critical CSS -->
<link rel="preload" href="/styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
<noscript><link rel="stylesheet" href="/styles.css"></noscript>
```

### JavaScript optimization

**Defer non-essential scripts:**
```html
<!-- Parser-blocking (avoid) -->
<script src="/critical.js"></script>

<!-- Deferred (preferred) -->
<script defer src="/app.js"></script>

<!-- Async (for independent scripts) -->
<script async src="/analytics.js"></script>

<!-- Module (deferred by default) -->
<script type="module" src="/app.mjs"></script>
```

**Code splitting patterns:**
```javascript
// Route-based splitting
const Dashboard = lazy(() => import('./Dashboard'));

// Component-based splitting
const HeavyChart = lazy(() => import('./HeavyChart'));

// Feature-based splitting
if (user.isPremium) {
  const PremiumFeatures = await import('./PremiumFeatures');
}
```

**Tree shaking best practices:**
```javascript
// ❌ Imports entire library
import _ from 'lodash';
_.debounce(fn, 300);

// ✅ Imports only what's needed
import debounce from 'lodash/debounce';
debounce(fn, 300);
```

## Image optimization

### Format selection
| Format | Use case | Browser support |
|--------|----------|-----------------|
| AVIF | Photos, best compression | 92%+ |
| WebP | Photos, good fallback | 97%+ |
| PNG | Graphics with transparency | Universal |
| SVG | Icons, logos, illustrations | Universal |

### Responsive images
```html
<picture>
  <!-- AVIF for modern browsers -->
  <source 
    type="image/avif"
    srcset="hero-400.avif 400w,
            hero-800.avif 800w,
            hero-1200.avif 1200w"
    sizes="(max-width: 600px) 100vw, 50vw">
  
  <!-- WebP fallback -->
  <source 
    type="image/webp"
    srcset="hero-400.webp 400w,
            hero-800.webp 800w,
            hero-1200.webp 1200w"
    sizes="(max-width: 600px) 100vw, 50vw">
  
  <!-- JPEG fallback -->
  <img 
    src="hero-800.jpg"
    srcset="hero-400.jpg 400w,
            hero-800.jpg 800w,
            hero-1200.jpg 1200w"
    sizes="(max-width: 600px) 100vw, 50vw"
    width="1200" 
    height="600"
    alt="Hero image"
    loading="lazy"
    decoding="async">
</picture>
```

### LCP image priority
```html
<!-- Above-fold LCP image: eager loading, high priority -->
<img 
  src="hero.webp" 
  fetchpriority="high"
  loading="eager"
  decoding="sync"
  alt="Hero">

<!-- Below-fold images: lazy loading -->
<img 
  src="product.webp" 
  loading="lazy"
  decoding="async"
  alt="Product">
```

## Font optimization

### Loading strategy
```css
/* System font stack as fallback */
body {
  font-family: 'Custom Font', -apple-system, BlinkMacSystemFont, 
               'Segoe UI', Roboto, sans-serif;
}

/* Prevent invisible text */
@font-face {
  font-family: 'Custom Font';
  src: url('/fonts/custom.woff2') format('woff2');
  font-display: swap; /* or optional for non-critical */
  font-weight: 400;
  font-style: normal;
  unicode-range: U+0000-00FF; /* Subset to Latin */
}
```

### Preloading critical fonts
```html
<link rel="preload" href="/fonts/heading.woff2" as="font" type="font/woff2" crossorigin>
```

### Variable fonts
```css
/* One file instead of multiple weights */
@font-face {
  font-family: 'Inter';
  src: url('/fonts/Inter-Variable.woff2') format('woff2-variations');
  font-weight: 100 900;
  font-display: swap;
}
```

## Caching strategy

### Cache-Control headers
```
# HTML (short or no cache)
Cache-Control: no-cache, must-revalidate

# Static assets with hash (immutable)
Cache-Control: public, max-age=31536000, immutable

# Static assets without hash
Cache-Control: public, max-age=86400, stale-while-revalidate=604800

# API responses
Cache-Control: private, max-age=0, must-revalidate
```

### Service worker caching
```javascript
// Cache-first for static assets
self.addEventListener('fetch', (event) => {
  if (event.request.destination === 'image' ||
      event.request.destination === 'style' ||
      event.request.destination === 'script') {
    event.respondWith(
      caches.match(event.request).then((cached) => {
        return cached || fetch(event.request).then((response) => {
          const clone = response.clone();
          caches.open('static-v1').then((cache) => cache.put(event.request, clone));
          return response;
        });
      })
    );
  }
});
```

## Runtime performance

### Avoid layout thrashing
```javascript
// ❌ Forces multiple reflows
elements.forEach(el => {
  const height = el.offsetHeight; // Read
  el.style.height = height + 10 + 'px'; // Write
});

// ✅ Batch reads, then batch writes
const heights = elements.map(el => el.offsetHeight); // All reads
elements.forEach((el, i) => {
  el.style.height = heights[i] + 10 + 'px'; // All writes
});
```

### Debounce expensive operations
```javascript
function debounce(fn, delay) {
  let timeout;
  return (...args) => {
    clearTimeout(timeout);
    timeout = setTimeout(() => fn(...args), delay);
  };
}

// Debounce scroll/resize handlers
window.addEventListener('scroll', debounce(handleScroll, 100));
```

### Use requestAnimationFrame
```javascript
// ❌ May cause jank
setInterval(animate, 16);

// ✅ Synced with display refresh
function animate() {
  // Animation logic
  requestAnimationFrame(animate);
}
requestAnimationFrame(animate);
```

### Virtualize long lists
```javascript
// For lists > 100 items, render only visible items
// Use libraries like react-window, vue-virtual-scroller, or native CSS:
.virtual-list {
  content-visibility: auto;
  contain-intrinsic-size: 0 50px; /* Estimated item height */
}
```

### Smooth navigations with View Transitions

The [View Transitions API](https://developer.chrome.com/docs/web-platform/view-transitions) lets the browser cross-fade (or custom-animate) between two DOM states using a single GPU-composited snapshot — no double-render, no layout thrash, and the snapshot doesn't count toward CLS.

**Same-document (SPA-style) — Baseline 2026:**
```javascript
// Wrap the DOM mutation that swaps the view
function navigate(newView) {
  if (!document.startViewTransition) return swapDOM(newView);
  document.startViewTransition(() => swapDOM(newView));
}
```

**Cross-document (MPA-style) — Chromium-stable, progressive enhancement elsewhere:**
```css
/* On both source and destination pages */
@view-transition { navigation: auto; }
```
That's the entire integration — same-origin navigations now fade automatically. To opt specific elements into shared-element transitions (e.g. a thumbnail expanding into a hero), give them a matching `view-transition-name`:
```css
.product-thumb[data-id="42"], .product-hero { view-transition-name: product-42; }
```

Pair this with Speculation Rules (above) for instant + animated navigations.

## Third-party scripts

### Load strategies
```javascript
// ❌ Blocks main thread
<script src="https://analytics.example.com/script.js"></script>

// ✅ Async loading
<script async src="https://analytics.example.com/script.js"></script>

// ✅ Delay until interaction
<script>
document.addEventListener('DOMContentLoaded', () => {
  const observer = new IntersectionObserver((entries) => {
    if (entries[0].isIntersecting) {
      const script = document.createElement('script');
      script.src = 'https://widget.example.com/embed.js';
      document.body.appendChild(script);
      observer.disconnect();
    }
  });
  observer.observe(document.querySelector('#widget-container'));
});
</script>
```

### Facade pattern
```html
<!-- Show static placeholder until interaction -->
<div class="youtube-facade" 
     data-video-id="abc123" 
     onclick="loadYouTube(this)">
  <img src="/thumbnails/abc123.jpg" alt="Video title">
  <button aria-label="Play video">▶</button>
</div>
```

## Measurement

### Key metrics
| Metric | Target | Tool |
|--------|--------|------|
| LCP | < 2.5s | Lighthouse, CrUX |
| FCP | < 1.8s | Lighthouse |
| Speed Index | < 3.4s | Lighthouse |
| TBT | < 200ms | Lighthouse |
| TTI | < 3.8s | Lighthouse |

### Testing commands
```bash
# Lighthouse CLI
npx lighthouse https://example.com --output html --output-path report.html

# Web Vitals library
import {onLCP, onINP, onCLS} from 'web-vitals';
onLCP(console.log);
onINP(console.log);
onCLS(console.log);
```

## References

For Core Web Vitals specific optimizations, see [Core Web Vitals](../core-web-vitals/SKILL.md).