Identify and fix performance issues to create faster, smoother user experiences.

Assess Performance Issues

Understand current performance and identify problems:

1. Measure current state:

   • Core Web Vitals: LCP, FID/INP, CLS scores
   • Load time: Time to interactive, first contentful paint
   • Bundle size: JavaScript, CSS, image sizes
   • Runtime performance: Frame rate, memory usage, CPU usage
   • Network: Request count, payload sizes, waterfall

2. Identify bottlenecks:

   • What's slow? (Initial load? Interactions? Animations?)
   • What's causing it? (Large images? Expensive JavaScript? Layout thrashing?)
   • How bad is it? (Perceivable? Annoying? Blocking?)
   • Who's affected? (All users? Mobile only? Slow connections?)

CRITICAL: Measure before and after. Premature optimization wastes time. Optimize what actually matters.

Optimization Strategy

Create systematic improvement plan:

Loading Performance

Optimize Images:

• Use modern formats (WebP, AVIF)
• Proper sizing (don't load 3000px image for 300px display)
• Lazy loading for below-fold images
• Responsive images (srcset, picture element)
• Compress images (80-85% quality is usually imperceptible)
• Use CDN for faster delivery

<img 
  src="hero.webp"
  srcset="hero-400.webp 400w, hero-800.webp 800w, hero-1200.webp 1200w"
  sizes="(max-width: 400px) 400px, (max-width: 800px) 800px, 1200px"
  loading="lazy"
  alt="Hero image"
/>

Reduce JavaScript Bundle:

• Code splitting (route-based, component-based)
• Tree shaking (remove unused code)
• Remove unused dependencies
• Lazy load non-critical code
• Use dynamic imports for large components

// Lazy load heavy component
const HeavyChart = lazy(() => import('./HeavyChart'));

Optimize CSS:

• Remove unused CSS
• Critical CSS inline, rest async
• Minimize CSS files
• Use CSS containment for independent regions

Optimize Fonts:

• Use font-display: swap or optional
• Subset fonts (only characters you need)
• Preload critical fonts
• Use system fonts when appropriate
• Limit font weights loaded

@font-face {
  font-family: 'CustomFont';
  src: url('/fonts/custom.woff2') format('woff2');
  font-display: swap; /* Show fallback immediately */
  unicode-range: U+0020-007F; /* Basic Latin only */
}

Optimize Loading Strategy:

• Critical resources first (async/defer non-critical)
• Preload critical assets
• Prefetch likely next pages
• Service worker for offline/caching
• HTTP/2 or HTTP/3 for multiplexing

Rendering Performance

Avoid Layout Thrashing:

// ❌ Bad: Alternating reads and writes (causes reflows)
elements.forEach(el => {
  const height = el.offsetHeight; // Read (forces layout)
  el.style.height = height * 2; // Write
});

// ✅ Good: Batch reads, then batch writes
const heights = elements.map(el => el.offsetHeight); // All reads
elements.forEach((el, i) => {
  el.style.height = heights[i] * 2; // All writes
});

Optimize Rendering:

• Use CSS contain property for independent regions
• Minimize DOM depth (flatter is faster)
• Reduce DOM size (fewer elements)
• Use content-visibility: auto for long lists
• Virtual scrolling for very long lists (react-window, react-virtualized)

Reduce Paint & Composite:

• Use transform and opacity for animations (GPU-accelerated)
• Avoid animating layout properties (width, height, top, left)
• Use will-change sparingly for known expensive operations
• Minimize paint areas (smaller is faster)

Animation Performance

GPU Acceleration:

/* ✅ GPU-accelerated (fast) */
.animated {
  transform: translateX(100px);
  opacity: 0.5;
}

/* ❌ CPU-bound (slow) */
.animated {
  left: 100px;
  width: 300px;
}

Smooth 60fps:

• Target 16ms per frame (60fps)
• Use requestAnimationFrame for JS animations
• Debounce/throttle scroll handlers
• Use CSS animations when possible
• Avoid long-running JavaScript during animations

Intersection Observer:

// Efficiently detect when elements enter viewport
const observer = new IntersectionObserver((entries) => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      // Element is visible, lazy load or animate
    }
  });
});

React/Framework Optimization

React-specific:

• Use memo() for expensive components
• useMemo() and useCallback() for expensive computations
• Virtualize long lists
• Code split routes
• Avoid inline function creation in render
• Use React DevTools Profiler

Framework-agnostic:

• Minimize re-renders
• Debounce expensive operations
• Memoize computed values
• Lazy load routes and components

Network Optimization

Reduce Requests:

• Combine small files
• Use SVG sprites for icons
• Inline small critical assets
• Remove unused third-party scripts

Optimize APIs:

• Use pagination (don't load everything)
• GraphQL to request only needed fields
• Response compression (gzip, brotli)
• HTTP caching headers
• CDN for static assets

Optimize for Slow Connections:

• Adaptive loading based on connection (navigator.connection)
• Optimistic UI updates
• Request prioritization
• Progressive enhancement

Core Web Vitals Optimization

Largest Contentful Paint (LCP < 2.5s)

• Optimize hero images
• Inline critical CSS
• Preload key resources
• Use CDN
• Server-side rendering

First Input Delay (FID < 100ms) / INP (< 200ms)

• Break up long tasks
• Defer non-critical JavaScript
• Use web workers for heavy computation
• Reduce JavaScript execution time

Cumulative Layout Shift (CLS < 0.1)

• Set dimensions on images and videos
• Don't inject content above existing content
• Use aspect-ratio CSS property
• Reserve space for ads/embeds
• Avoid animations that cause layout shifts

/* Reserve space for image */
.image-container {
  aspect-ratio: 16 / 9;
}

Performance Monitoring

Tools to use:

• Chrome DevTools (Lighthouse, Performance panel)
• WebPageTest
• Core Web Vitals (Chrome UX Report)
• Bundle analyzers (webpack-bundle-analyzer)
• Performance monitoring (Sentry, DataDog, New Relic)

Key metrics:

• LCP, FID/INP, CLS (Core Web Vitals)
• Time to Interactive (TTI)
• First Contentful Paint (FCP)
• Total Blocking Time (TBT)
• Bundle size
• Request count

IMPORTANT: Measure on real devices with real network conditions. Desktop Chrome with fast connection isn't representative.

NEVER:

• Optimize without measuring (premature optimization)
• Sacrifice accessibility for performance
• Break functionality while optimizing
• Use will-change everywhere (creates new layers, uses memory)
• Lazy load above-fold content
• Optimize micro-optimizations while ignoring major issues (optimize the biggest bottleneck first)
• Forget about mobile performance (often slower devices, slower connections)

Verify Improvements

Test that optimizations worked:

• Before/after metrics: Compare Lighthouse scores
• Real user monitoring: Track improvements for real users
• Different devices: Test on low-end Android, not just flagship iPhone
• Slow connections: Throttle to 3G, test experience
• No regressions: Ensure functionality still works
• User perception: Does it feel faster?

Remember: Performance is a feature. Fast experiences feel more responsive, more polished, more professional. Optimize systematically, measure ruthlessly, and prioritize user-perceived performance.