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.

Resource Loading

Preconnect
Preload
Defer CSS

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

<!-- 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>

<!-- 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

<!-- 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>

// 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');
}

// ❌ 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 */
}

<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;
}

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

elements.forEach(el => {
  const height = el.offsetHeight; // Read
  el.style.height = height + 10 + 'px'; // Write
});

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

setInterval(animate, 16);

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 */
}

Third-Party Scripts

Load Strategies

<script src="https://analytics.example.com/script.js"></script>

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

<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);

For Core Web Vitals specific optimizations, see Core Web Vitals.

Overview

Audit Skills

Performance

Quality & Standards

Performance Optimization

Performance Optimization

How it Works

Performance Budget

Critical Rendering Path

Server Response

Resource Loading

JavaScript Optimization

Image Optimization

Format Selection

Responsive Images

LCP Image Priority

Font Optimization

Loading Strategy

Caching Strategy

Cache-Control Headers

Service Worker Caching

Runtime Performance

Avoid Layout Thrashing

Debounce Expensive Operations

Use requestAnimationFrame

Virtualize Long Lists

Third-Party Scripts

Load Strategies

Facade Pattern

Measurement

Key Metrics

Testing Commands

​Performance Optimization

​How it Works

​Performance Budget

​Critical Rendering Path

​Server Response

​Resource Loading

​JavaScript Optimization

​Image Optimization

​Format Selection

​Responsive Images

​LCP Image Priority

​Font Optimization

​Loading Strategy

​Caching Strategy

​Cache-Control Headers

​Service Worker Caching

​Runtime Performance

​Avoid Layout Thrashing

​Debounce Expensive Operations

​Use requestAnimationFrame

​Virtualize Long Lists

​Third-Party Scripts

​Load Strategies

​Facade Pattern

​Measurement

​Key Metrics

​Testing Commands

Performance Optimization

How it Works

Performance Budget

Critical Rendering Path

Server Response

Resource Loading

JavaScript Optimization

Image Optimization

Format Selection

Responsive Images

LCP Image Priority

Font Optimization

Loading Strategy

Caching Strategy

Cache-Control Headers

Service Worker Caching

Runtime Performance

Avoid Layout Thrashing

Debounce Expensive Operations

Use requestAnimationFrame

Virtualize Long Lists

Third-Party Scripts

Load Strategies

Facade Pattern

Measurement

Key Metrics

Testing Commands