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

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 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:

<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com" crossorigin>

Preload critical resources:

<!-- 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:

<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:

<!-- 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:

<!-- 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:

// 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:

// ❌ 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

Responsive images

<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">