performance

SKILL.md

Performance optimization

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

How it works

Identify performance bottlenecks in code and assets

Prioritize by impact on Core Web Vitals

Provide specific optimizations with code examples

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 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 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

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

<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

<!-- 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

/* 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

<link rel="preload" href="/fonts/heading.woff2" as="font" type="font/woff2" crossorigin>

Variable fonts

/* 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

// 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

// ❌ 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

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

// ❌ May cause jank
setInterval(animate, 16);

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

Virtualize long lists

// 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 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:

// 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:

/* 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:

.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

// ❌ 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

<!-- 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

# 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.