Web Performance Optimization

Overview

Help developers optimize website and web application performance to improve user experience, SEO rankings, and conversion rates. This skill provides systematic approaches to measure, analyze, and improve loading speed, runtime performance, and Core Web Vitals metrics.

When to Use This Skill

• Use when website or app is loading slowly
• Use when optimizing for Core Web Vitals (LCP, FID, CLS)
• Use when reducing JavaScript bundle size
• Use when improving Time to Interactive (TTI)
• Use when optimizing images and assets
• Use when implementing caching strategies
• Use when debugging performance bottlenecks
• Use when preparing for performance audits

How It Works

Step 1: Measure Current Performance

I'll help you establish baseline metrics:

• Run Lighthouse audits
• Measure Core Web Vitals (LCP, FID, CLS)
• Check bundle sizes
• Analyze network waterfall
• Identify performance bottlenecks

Step 2: Identify Issues

Analyze performance problems:

• Large JavaScript bundles
• Unoptimized images
• Render-blocking resources
• Slow server response times
• Missing caching headers
• Layout shifts
• Long tasks blocking main thread

Step 3: Prioritize Optimizations

Focus on high-impact improvements:

• Critical rendering path optimization
• Code splitting and lazy loading
• Image optimization
• Caching strategies
• Third-party script optimization

Step 4: Implement Optimizations

Apply performance improvements:

• Optimize assets (images, fonts, CSS, JS)
• Implement code splitting
• Add caching headers
• Lazy load non-critical resources
• Optimize critical rendering path

Step 5: Verify Improvements

Measure impact of changes:

• Re-run Lighthouse audits
• Compare before/after metrics
• Monitor real user metrics (RUM)
• Test on different devices and networks

Examples

Example 1: Optimizing Core Web Vitals

## Performance Audit Results

### Current Metrics (Before Optimization)
- **LCP (Largest Contentful Paint):** 4.2s ❌ (should be < 2.5s)
- **FID (First Input Delay):** 180ms ❌ (should be < 100ms)
- **CLS (Cumulative Layout Shift):** 0.25 ❌ (should be < 0.1)
- **Lighthouse Score:** 62/100

### Issues Identified

1. **LCP Issue:** Hero image (2.5MB) loads slowly
2. **FID Issue:** Large JavaScript bundle (850KB) blocks main thread
3. **CLS Issue:** Images without dimensions cause layout shifts

### Optimization Plan

#### Fix LCP (Largest Contentful Paint)

**Problem:** Hero image is 2.5MB and loads slowly

**Solutions:**
\`\`\`html
<!-- Before: Unoptimized image -->
<img src="/hero.jpg" alt="Hero">

<!-- After: Optimized with modern formats -->
<picture>
  <source srcset="/hero.avif" type="image/avif">
  <source srcset="/hero.webp" type="image/webp">
  <img 
    src="/hero.jpg" 
    alt="Hero"
    width="1200" 
    height="600"
    loading="eager"
    fetchpriority="high"
  >
</picture>
\`\`\`

**Additional optimizations:**
- Compress image to < 200KB
- Use CDN for faster delivery
- Preload hero image: `<link rel="preload" as="image" href="/hero.avif">`

#### Fix FID (First Input Delay)

**Problem:** 850KB JavaScript bundle blocks main thread

**Solutions:**

1. **Code Splitting:**
\`\`\`javascript
// Before: Everything in one bundle
import { HeavyComponent } from './HeavyComponent';
import { Analytics } from './analytics';
import { ChatWidget } from './chat';

// After: Lazy load non-critical code
const HeavyComponent = lazy(() => import('./HeavyComponent'));
const ChatWidget = lazy(() => import('./chat'));

// Load analytics after page interactive
if (typeof window !== 'undefined') {
  window.addEventListener('load', () => {
    import('./analytics').then(({ Analytics }) => {
      Analytics.init();
    });
  });
}
\`\`\`

2. **Remove Unused Dependencies:**
\`\`\`bash
# Analyze bundle
npx webpack-bundle-analyzer

# Remove unused packages
npm uninstall moment  # Use date-fns instead (smaller)
npm install date-fns
\`\`\`

3. **Defer Non-Critical Scripts:**
\`\`\`html
<!-- Before: Blocks rendering -->
<script src="/analytics.js"></script>

<!-- After: Deferred -->
<script src="/analytics.js" defer></script>
\`\`\`

#### Fix CLS (Cumulative Layout Shift)

**Problem:** Images without dimensions cause layout shifts

**Solutions:**
\`\`\`html
<!-- Before: No dimensions -->
<img src="/product.jpg" alt="Product">

<!-- After: With dimensions -->
<img 
  src="/product.jpg" 
  alt="Product"
  width="400" 
  height="300"
  style="aspect-ratio: 4/3;"
>
\`\`\`

**For dynamic content:**
\`\`\`css
/* Reserve space for content that loads later */
.skeleton-loader {
  min-height: 200px;
  background: linear-gradient(90deg, #f0f0f0 25%, #e0e0e0 50%, #f0f0f0 75%);
  background-size: 200% 100%;
  animation: loading 1.5s infinite;
}

@keyframes loading {
  0% { background-position: 200% 0; }
  100% { background-position: -200% 0; }
}
\`\`\`

### Results After Optimization

- **LCP:** 1.8s ✅ (improved by 57%)
- **FID:** 45ms ✅ (improved by 75%)
- **CLS:** 0.05 ✅ (improved by 80%)
- **Lighthouse Score:** 94/100 ✅

Example 2: Reducing JavaScript Bundle Size

## Bundle Size Optimization

### Current State
- **Total Bundle:** 850KB (gzipped: 280KB)
- **Main Bundle:** 650KB
- **Vendor Bundle:** 200KB
- **Load Time (3G):** 8.2s

### Analysis

\`\`\`bash
# Analyze bundle composition
npx webpack-bundle-analyzer dist/stats.json
\`\`\`

**Findings:**
1. Moment.js: 67KB (can replace with date-fns: 12KB)
2. Lodash: 72KB (using entire library, only need 5 functions)
3. Unused code: ~150KB of dead code
4. No code splitting: Everything in one bundle

### Optimization Steps

#### 1. Replace Heavy Dependencies

\`\`\`bash
# Remove moment.js (67KB) → Use date-fns (12KB)
npm uninstall moment
npm install date-fns

# Before
import moment from 'moment';
const formatted = moment(date).format('YYYY-MM-DD');

# After
import { format } from 'date-fns';
const formatted = format(date, 'yyyy-MM-dd');
\`\`\`

**Savings:** 55KB

#### 2. Use Lodash Selectively

\`\`\`javascript
// Before: Import entire library (72KB)
import _ from 'lodash';
const unique = _.uniq(array);

// After: Import only what you need (5KB)
import uniq from 'lodash/uniq';
const unique = uniq(array);

// Or use native methods
const unique = [...new Set(array)];
\`\`\`

**Savings:** 67KB

#### 3. Implement Code Splitting

\`\`\`javascript
// Next.js example
import dynamic from 'next/dynamic';

// Lazy load heavy components
const Chart = dynamic(() => import('./Chart'), {
  loading: () => <div>Loading chart...</div>,
  ssr: false
});

const AdminPanel = dynamic(() => import('./AdminPanel'), {
  loading: () => <div>Loading...</div>
});

// Route-based code splitting (automatic in Next.js)
// pages/admin.js - Only loaded when visiting /admin
// pages/dashboard.js - Only loaded when visiting /dashboard
\`\`\`

#### 4. Remove Dead Code

\`\`\`javascript
// Enable tree shaking in webpack.config.js
module.exports = {
  mode: 'production',
  optimization: {
    usedExports: true,
    sideEffects: false
  }
};

// In package.json
{
  "sideEffects": false
}
\`\`\`

#### 5. Optimize Third-Party Scripts

\`\`\`html
<!-- Before: Loads immediately -->
<script src="https://analytics.com/script.js"></script>

<!-- After: Load after page interactive -->
<script>
  window.addEventListener('load', () => {
    const script = document.createElement('script');
    script.src = 'https://analytics.com/script.js';
    script.async = true;
    document.body.appendChild(script);
  });
</script>
\`\`\`

### Results

- **Total Bundle:** 380KB ✅ (reduced by 55%)
- **Main Bundle:** 180KB ✅
- **Vendor Bundle:** 80KB ✅
- **Load Time (3G):** 3.1s ✅ (improved by 62%)

Example 3: Image Optimization Strategy

## Image Optimization

### Current Issues
- 15 images totaling 12MB
- No modern formats (WebP, AVIF)
- No responsive images
- No lazy loading

### Optimization Strategy

#### 1. Convert to Modern Formats

\`\`\`bash
# Install image optimization tools
npm install sharp

# Conversion script (optimize-images.js)
const sharp = require('sharp');
const fs = require('fs');
const path = require('path');

async function optimizeImage(inputPath, outputDir) {
  const filename = path.basename(inputPath, path.extname(inputPath));
  
  // Generate WebP
  await sharp(inputPath)
    .webp({ quality: 80 })
    .toFile(path.join(outputDir, \`\${filename}.webp\`));
  
  // Generate AVIF (best compression)
  await sharp(inputPath)
    .avif({ quality: 70 })
    .toFile(path.join(outputDir, \`\${filename}.avif\`));
  
  // Generate optimized JPEG fallback
  await sharp(inputPath)
    .jpeg({ quality: 80, progressive: true })
    .toFile(path.join(outputDir, \`\${filename}.jpg\`));
}

// Process all images
const images = fs.readdirSync('./images');
images.forEach(img => {
  optimizeImage(\`./images/\${img}\`, './images/optimized');
});
\`\`\`

#### 2. Implement Responsive Images

\`\`\`html
<!-- Responsive images with modern formats -->
<picture>
  <!-- AVIF for browsers that support it (best compression) -->
  <source 
    srcset="
      /images/hero-400.avif 400w,
      /images/hero-800.avif 800w,
      /images/hero-1200.avif 1200w
    "
    type="image/avif"
    sizes="(max-width: 768px) 100vw, 50vw"
  >
  
  <!-- WebP for browsers that support it -->
  <source 
    srcset="
      /images/hero-400.webp 400w,
      /images/hero-800.webp 800w,
      /images/hero-1200.webp 1200w
    "
    type="image/webp"
    sizes="(max-width: 768px) 100vw, 50vw"
  >
  
  <!-- JPEG fallback -->
  <img 
    src="/images/hero-800.jpg"
    srcset="
      /images/hero-400.jpg 400w,
      /images/hero-800.jpg 800w,
      /images/hero-1200.jpg 1200w
    "
    sizes="(max-width: 768px) 100vw, 50vw"
    alt="Hero image"
    width="1200"
    height="600"
    loading="lazy"
  >
</picture>
\`\`\`

#### 3. Lazy Loading

\`\`\`html
<!-- Native lazy loading -->
<img 
  src="/image.jpg" 
  alt="Description"
  loading="lazy"
  width="800"
  height="600"
>

<!-- Eager loading for above-the-fold images -->
<img 
  src="/hero.jpg" 
  alt="Hero"
  loading="eager"
  fetchpriority="high"
>
\`\`\`

#### 4. Next.js Image Component

\`\`\`javascript
import Image from 'next/image';

// Automatic optimization
<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority  // For above-the-fold images
  quality={80}
/>

// Lazy loaded
<Image
  src="/product.jpg"
  alt="Product"
  width={400}
  height={300}
  loading="lazy"
/>
\`\`\`

### Results

| Metric | Before | After | Improvement |
|--------|--------|-------|-------------|
| Total Image Size | 12MB | 1.8MB | 85% reduction |
| LCP | 4.5s | 1.6s | 64% faster |
| Page Load (3G) | 18s | 4.2s | 77% faster |

Best Practices

✅ Do This

• Measure First - Always establish baseline metrics before optimizing
• Use Lighthouse - Run audits regularly to track progress
• Optimize Images - Use modern formats (WebP, AVIF) and responsive images
• Code Split - Break large bundles into smaller chunks
• Lazy Load - Defer non-critical resources
• Cache Aggressively - Set proper cache headers for static assets
• Minimize Main Thread Work - Keep JavaScript execution under 50ms chunks
• Preload Critical Resources - Use <link rel="preload"> for critical assets
• Use CDN - Serve static assets from CDN for faster delivery
• Monitor Real Users - Track Core Web Vitals from real users

❌ Don't Do This

• Don't Optimize Blindly - Measure first, then optimize
• Don't Ignore Mobile - Test on real mobile devices and slow networks
• Don't Block Rendering - Avoid render-blocking CSS and JavaScript
• Don't Load Everything Upfront - Lazy load non-critical resources
• Don't Forget Dimensions - Always specify image width/height
• Don't Use Synchronous Scripts - Use async or defer attributes
• Don't Ignore Third-Party Scripts - They often cause performance issues
• Don't Skip Compression - Always compress and minify assets

Common Pitfalls

Problem: Optimized for Desktop but Slow on Mobile

Symptoms: Good Lighthouse score on desktop, poor on mobile Solution:

• Test on real mobile devices
• Use Chrome DevTools mobile throttling
• Optimize for 3G/4G networks
• Reduce JavaScript execution time

# Test with throttling
lighthouse https://yoursite.com --throttling.cpuSlowdownMultiplier=4

Problem: Large JavaScript Bundle

Symptoms: Long Time to Interactive (TTI), high FID Solution:

• Analyze bundle with webpack-bundle-analyzer
• Remove unused dependencies
• Implement code splitting
• Lazy load non-critical code

# Analyze bundle
npx webpack-bundle-analyzer dist/stats.json

Problem: Images Causing Layout Shifts

Symptoms: High CLS score, content jumping Solution:

• Always specify width and height
• Use aspect-ratio CSS property
• Reserve space with skeleton loaders

img {
  aspect-ratio: 16 / 9;
  width: 100%;
  height: auto;
}

Problem: Slow Server Response Time

Symptoms: High TTFB (Time to First Byte) Solution:

• Implement server-side caching
• Use CDN for static assets
• Optimize database queries
• Consider static site generation (SSG)

// Next.js: Static generation
export async function getStaticProps() {
  const data = await fetchData();
  return {
    props: { data },
    revalidate: 60 // Regenerate every 60 seconds
  };
}

Performance Checklist

Images

• Convert to modern formats (WebP, AVIF)
• Implement responsive images
• Add lazy loading
• Specify dimensions (width/height)
• Compress images (< 200KB each)
• Use CDN for delivery

JavaScript

• Bundle size < 200KB (gzipped)
• Implement code splitting
• Lazy load non-critical code
• Remove unused dependencies
• Minify and compress
• Use async/defer for scripts

CSS

• Inline critical CSS
• Defer non-critical CSS
• Remove unused CSS
• Minify CSS files
• Use CSS containment

Caching

• Set cache headers for static assets
• Implement service worker
• Use CDN caching
• Cache API responses
• Version static assets

Core Web Vitals

• LCP < 2.5s
• FID < 100ms
• CLS < 0.1
• TTFB < 600ms
• TTI < 3.8s

Performance Tools

Measurement Tools

• Lighthouse - Comprehensive performance audit
• WebPageTest - Detailed waterfall analysis
• Chrome DevTools - Performance profiling
• PageSpeed Insights - Real user metrics
• Web Vitals Extension - Monitor Core Web Vitals

Analysis Tools

• webpack-bundle-analyzer - Visualize bundle composition
• source-map-explorer - Analyze bundle size
• Bundlephobia - Check package sizes before installing
• ImageOptim - Image compression tool

Monitoring Tools

• Google Analytics - Track Core Web Vitals
• Sentry - Performance monitoring
• New Relic - Application performance monitoring
• Datadog - Real user monitoring

Related Skills

• @react-best-practices - React performance patterns
• @frontend-dev-guidelines - Frontend development standards
• @systematic-debugging - Debug performance issues
• @senior-architect - Architecture for performance

Additional Resources

• Web.dev Performance
• Core Web Vitals
• Lighthouse Documentation
• MDN Performance Guide
• Next.js Performance
• Image Optimization Guide

---

Pro Tip: Focus on Core Web Vitals (LCP, FID, CLS) first - they have the biggest impact on user experience and SEO rankings!

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.