Optimize website and web application performance including loading speed, Core Web Vitals, bundle size, caching strategies, and runtime performance
npxskills add davila7/claude-code-templates--skill web-performance-optimizationLoading…
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.
I'll help you establish baseline metrics:
Analyze performance problems:
Focus on high-impact improvements:
Apply performance improvements:
Measure impact of changes:
## 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 ✅
## 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%)
## 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 |
<link rel="preload"> for critical assetsSymptoms: Good Lighthouse score on desktop, poor on mobile Solution:
# Test with throttling
lighthouse https://yoursite.com --throttling.cpuSlowdownMultiplier=4
Symptoms: Long Time to Interactive (TTI), high FID Solution:
# Analyze bundle
npx webpack-bundle-analyzer dist/stats.json
Symptoms: High CLS score, content jumping Solution:
img {
aspect-ratio: 16 / 9;
width: 100%;
height: auto;
}
Symptoms: High TTFB (Time to First Byte) Solution:
// Next.js: Static generation
export async function getStaticProps() {
const data = await fetchData();
return {
props: { data },
revalidate: 60 // Regenerate every 60 seconds
};
}
@react-best-practices - React performance patterns@frontend-dev-guidelines - Frontend development standards@systematic-debugging - Debug performance issues@senior-architect - Architecture for performancePro Tip: Focus on Core Web Vitals (LCP, FID, CLS) first - they have the biggest impact on user experience and SEO rankings!
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