web-performance

Web Performance

Systematic approach to measuring, analyzing, and optimizing web application performance.

Core Web Vitals

Metric	Target	What It Measures
LCP (Largest Contentful Paint)	< 2.5s	Loading performance
FID (First Input Delay)	< 100ms	Interactivity
CLS (Cumulative Layout Shift)	< 0.1	Visual stability
INP (Interaction to Next Paint)	< 200ms	Responsiveness

Optimization Strategies

1. JavaScript Optimization

// Dynamic imports — code splitting
const Chart = dynamic(() => import('./Chart'), {
  loading: () => <ChartSkeleton />,
  ssr: false, // If chart needs browser APIs
});

// Bundle analysis
// next.config.js
const withBundleAnalyzer = require('@next/bundle-analyzer')({
  enabled: process.env.ANALYZE === 'true',
});
module.exports = withBundleAnalyzer({});

2. Image Optimization

// Next.js Image component
import Image from 'next/image';

<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority // Above-the-fold images
  placeholder="blur"
  blurDataURL="data:image/webp;base64,..."
/>

// Responsive images with srcSet
<picture>
  <source media="(min-width: 1024px)" srcSet="/hero-lg.webp" />
  <source media="(min-width: 640px)" srcSet="/hero-md.webp" />
  <img src="/hero-sm.webp" alt="Hero" loading="lazy" />
</picture>

3. Caching Strategy

// Service Worker caching strategies
// Cache-First for static assets
self.addEventListener('fetch', (event) => {
  if (event.request.url.includes('/static/')) {
    event.respondWith(cacheFirst(event.request));
  }
  // Network-First for API calls
  if (event.request.url.includes('/api/')) {
    event.respondWith(networkFirst(event.request));
  }
});

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Common Mistakes

1. Optimizing before measuring: Always measure first. Use Lighthouse, WebPageTest, and RUM data.
2. Ignoring mobile performance: Test on real mobile devices, not just desktop DevTools.
3. Blocking rendering with JavaScript: Defer non-critical JS. Use async/defer on scripts.
4. Not optimizing images: Use WebP/AVIF, responsive images, and lazy loading.
5. Missing performance budgets: Set budgets in your CI pipeline and fail builds that exceed them.