---

name: web-performance-optimization description: Optimizes web application performance through code splitting, lazy loading, caching strategies, and Core Web Vitals monitoring. Use when improving page load times, implementing service workers, or reducing bundle sizes. license: MIT

Web Performance Optimization

Overview

Implement performance optimization strategies including lazy loading, code splitting, caching, compression, and monitoring to improve Core Web Vitals and user experience.

When to Use

• Slow page load times
• High Largest Contentful Paint (LCP)
• Large bundle sizes
• Frequent Cumulative Layout Shift (CLS)
• Mobile performance issues

Code Splitting (React)

import { lazy, Suspense } from 'react';
import { Routes, Route } from 'react-router-dom';

const Home = lazy(() => import('./pages/Home'));
const Dashboard = lazy(() => import('./pages/Dashboard'));
const Settings = lazy(() => import('./pages/Settings'));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/settings" element={<Settings />} />
      </Routes>
    </Suspense>
  );
}

Webpack Bundle Optimization

// webpack.config.js
module.exports = {
  optimization: {
    splitChunks: {
      chunks: 'all',
      cacheGroups: {
        vendor: {
          test: /[\\/]node_modules[\\/]/,
          name: 'vendors',
          chunks: 'all'
        }
      }
    }
  }
};

Image Optimization

<picture>
  <source srcset="image.webp" type="image/webp">
  <source srcset="image.jpg" type="image/jpeg">
  <img
    src="image.jpg"
    srcset="image-400.jpg 400w, image-800.jpg 800w, image-1200.jpg 1200w"
    sizes="(max-width: 600px) 100vw, 50vw"
    loading="lazy"
    decoding="async"
    alt="Description"
  >
</picture>

Service Worker Caching

// sw.js
const CACHE_NAME = 'app-v1';
const ASSETS = ['/', '/index.html', '/main.js', '/styles.css'];

self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(CACHE_NAME).then(cache => cache.addAll(ASSETS))
  );
});

self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.match(event.request).then(cached => {
      return cached || fetch(event.request).then(response => {
        return caches.open(CACHE_NAME).then(cache => {
          cache.put(event.request, response.clone());
          return response;
        });
      });
    })
  );
});

Core Web Vitals Monitoring

// Track LCP, CLS, INP (Note: INP replaced FID as of March 2024)
// sendToAnalytics is a placeholder function that sends metrics to your analytics endpoint
// Expected signature: sendToAnalytics({ metric: string, value: number }) => void
// Example implementation:
function sendToAnalytics({ metric, value }) {
  // Replace with your analytics implementation (e.g., Google Analytics, Segment)
  fetch('/api/analytics', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ metric, value, timestamp: Date.now() })
  });
}

// Largest Contentful Paint (LCP)
new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    console.log(`LCP: ${entry.startTime}ms`);
    sendToAnalytics({ metric: 'LCP', value: entry.startTime });
  }
}).observe({ type: 'largest-contentful-paint', buffered: true });

// Cumulative Layout Shift (CLS)
new PerformanceObserver((list) => {
  let cls = 0;
  for (const entry of list.getEntries()) {
    if (!entry.hadRecentInput) cls += entry.value;
  }
  sendToAnalytics({ metric: 'CLS', value: cls });
}).observe({ type: 'layout-shift', buffered: true });

// Interaction to Next Paint (INP) - replaces FID
new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    // INP measures responsiveness - duration of slowest interaction
    const inp = entry.processingEnd - entry.processingStart;
    console.log(`INP: ${inp}ms`);
    sendToAnalytics({ metric: 'INP', value: inp });
  }
}).observe({ type: 'event', buffered: true }); // 'event' captures interaction events

Performance Targets

Metric	Good	Needs Improvement
LCP	<2.5s	2.5-4s
INP	<200ms	200-500ms
CLS	<0.1	0.1-0.25
TTI	<3.8s	3.8-7.3s

Note: INP (Interaction to Next Paint) replaced FID (First Input Delay) as a Core Web Vital in March 2024. INP provides a more comprehensive measure of page responsiveness by capturing the full duration of interactions, not just the input delay.

Compression (Nginx)

gzip on;
gzip_types text/plain text/css application/json application/javascript;
gzip_min_length 1000;
gzip_comp_level 6;

Best Practices

• Minimize bundle size with code splitting
• Optimize images with appropriate formats
• Implement lazy loading strategically
• Use HTTP caching headers
• Enable gzip/brotli compression
• Monitor Core Web Vitals continuously
• Implement service workers
• Defer non-critical JavaScript
• Optimize critical rendering path
• Test on real devices and networks

Optimization Checklist

• Enable code splitting for routes
• Lazy load below-fold components
• Optimize and compress images
• Implement service worker caching
• Enable gzip/brotli compression
• Monitor Core Web Vitals
• Minimize render-blocking resources

Additional Configuration

See references/compression-monitoring.md for:

• Webpack compression plugin setup
• Apache .htaccess compression config
• TTFB monitoring implementation
• Puppeteer automation for measurement

See references/typescript-advanced.md for:

• TypeScript lazyLoad utility
• TypeScript image component
• Advanced service worker with offline fallback
• TerserPlugin configuration
• Complete PerformanceMetrics interface

Tools

• Lighthouse / PageSpeed Insights
• WebPageTest
• Chrome DevTools Performance tab
• web-vitals npm package

Resources

• Web Vitals
• Google PageSpeed Insights
• Lighthouse
• WebPageTest
• Performance API