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".
npxskills add addyosmani/web-quality-skills--skill performanceLoading…
Deep performance optimization based on Lighthouse performance audits. Focuses on loading speed, runtime efficiency, and resource optimization.
| 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 |
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.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>
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);
| 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 |
<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>
<!-- 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">
/* 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 */
}
<link rel="preload" href="/fonts/heading.woff2" as="font" type="font/woff2" crossorigin>
/* 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;
}
# 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
// 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;
});
})
);
}
});
// ❌ 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
});
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));
// ❌ May cause jank
setInterval(animate, 16);
// ✅ Synced with display refresh
function animate() {
// Animation logic
requestAnimationFrame(animate);
}
requestAnimationFrame(animate);
// 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 */
}
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.
// ❌ 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>
<!-- 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>
| Metric | Target | Tool |
|---|---|---|
| LCP | < 2.5s | Lighthouse, CrUX |
| FCP | < 1.8s | Lighthouse |
| Speed Index | < 3.4s | Lighthouse |
| TBT | < 200ms | Lighthouse |
| TTI | < 3.8s | Lighthouse |
# 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);
For Core Web Vitals specific optimizations, see Core Web Vitals.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).