Performance Optimization

When to use this skill

• Slow page loads: low Lighthouse score
• Slow rendering: delayed user interactions
• Large bundle size: increased download time
• Slow queries: database bottlenecks

Instructions

Step 1: Measure performance

Lighthouse (Chrome DevTools):

# CLI
npm install -g lighthouse
lighthouse https://example.com --view

# Automate in CI
lighthouse https://example.com --output=json --output-path=./report.json

Measure Web Vitals (React):

import { getCLS, getFID, getFCP, getLCP, getTTFB } from 'web-vitals';

function sendToAnalytics(metric: any) {
  // Send to Google Analytics, Datadog, etc.
  console.log(metric);
}

getCLS(sendToAnalytics);
getFID(sendToAnalytics);
getFCP(sendToAnalytics);
getLCP(sendToAnalytics);
getTTFB(sendToAnalytics);

Step 2: Optimize React

React.memo (prevent unnecessary re-renders):

// ❌ Bad: child re-renders whenever the parent re-renders
function ExpensiveComponent({ data }: { data: Data }) {
  return <div>{/* complex rendering */}</div>;
}

// ✅ Good: re-render only when props change
const ExpensiveComponent = React.memo(({ data }: { data: Data }) => {
  return <div>{/* complex rendering */}</div>;
});

useMemo & useCallback:

function ProductList({ products, category }: Props) {
  // ✅ Memoize filtered results
  const filteredProducts = useMemo(() => {
    return products.filter(p => p.category === category);
  }, [products, category]);

  // ✅ Memoize callback
  const handleAddToCart = useCallback((id: string) => {
    addToCart(id);
  }, []);

  return (
    <div>
      {filteredProducts.map(product => (
        <ProductCard key={product.id} product={product} onAdd={handleAddToCart} />
      ))}
    </div>
  );
}

Lazy Loading & Code Splitting:

import { lazy, Suspense } from 'react';

// ✅ Route-based code splitting
const Dashboard = lazy(() => import('./pages/Dashboard'));
const Profile = lazy(() => import('./pages/Profile'));
const Settings = lazy(() => import('./pages/Settings'));

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

// ✅ Component-based lazy loading
const HeavyChart = lazy(() => import('./components/HeavyChart'));

function Dashboard() {
  return (
    <div>
      <h1>Dashboard</h1>
      <Suspense fallback={<Skeleton />}>
        <HeavyChart data={data} />
      </Suspense>
    </div>
  );
}

Step 3: Optimize bundle size

Webpack Bundle Analyzer:

npm install --save-dev webpack-bundle-analyzer

# package.json
{
  "scripts": {
    "analyze": "webpack-bundle-analyzer build/stats.json"
  }
}

Tree Shaking (remove unused code):

// ❌ Bad: import entire library
import _ from 'lodash';

// ✅ Good: import only what you need
import debounce from 'lodash/debounce';

Dynamic Imports:

// ✅ Load only when needed
button.addEventListener('click', async () => {
  const { default: Chart } = await import('chart.js');
  new Chart(ctx, config);
});

Step 4: Optimize images

Next.js Image component:

import Image from 'next/image';

function ProductImage() {
  return (
    <Image
      src="/product.jpg"
      alt="Product"
      width={500}
      height={500}
      priority  // for the LCP image
      placeholder="blur"  // blur placeholder
      sizes="(max-width: 768px) 100vw, 50vw"
    />
  );
}

Use WebP format:

<picture>
  <source srcset="image.webp" type="image/webp">
  <source srcset="image.jpg" type="image/jpeg">
  <img src="image.jpg" alt="Fallback">
</picture>

Step 5: Optimize database queries

Fix the N+1 query problem:

// ❌ Bad: N+1 queries
const posts = await db.post.findMany();
for (const post of posts) {
  const author = await db.user.findUnique({ where: { id: post.authorId } });
  // 101 queries (1 + 100)
}

// ✅ Good: JOIN or include
const posts = await db.post.findMany({
  include: {
    author: true
  }
});
// 1 query

Add indexes:

-- Identify slow queries
EXPLAIN ANALYZE SELECT * FROM users WHERE email = 'test@example.com';

-- Add index
CREATE INDEX idx_users_email ON users(email);

-- Composite index
CREATE INDEX idx_orders_user_date ON orders(user_id, created_at);

Caching (Redis):

async function getUserProfile(userId: string) {
  // 1. Check cache
  const cached = await redis.get(`user:${userId}`);
  if (cached) {
    return JSON.parse(cached);
  }

  // 2. Query DB
  const user = await db.user.findUnique({ where: { id: userId } });

  // 3. Store in cache (1 hour)
  await redis.setex(`user:${userId}`, 3600, JSON.stringify(user));

  return user;
}

Output format

Performance optimization checklist

## Frontend
- [ ] Prevent unnecessary re-renders with React.memo
- [ ] Use useMemo/useCallback appropriately
- [ ] Lazy loading & Code splitting
- [ ] Optimize images (WebP, lazy loading)
- [ ] Analyze and reduce bundle size

## Backend
- [ ] Remove N+1 queries
- [ ] Add database indexes
- [ ] Redis caching
- [ ] Compress API responses (gzip)
- [ ] Use a CDN

## Measurement
- [ ] Lighthouse score 90+
- [ ] LCP < 2.5s
- [ ] FID < 100ms
- [ ] CLS < 0.1

Constraints

Required rules (MUST)

1. Measure first: profile, don't guess
2. Incremental improvements: optimize one thing at a time
3. Performance monitoring: track continuously

Prohibited items (MUST NOT)

1. Premature optimization: don't optimize when there is no bottleneck
2. Sacrificing readability: don't make code complex for performance

Best practices

1. 80/20 rule: 80% improvement with 20% effort
2. User-centered: focus on improving real user experience
3. Automation: performance regression tests in CI

References

• web.dev/vitals
• React Optimization
• Webpack Bundle Analyzer

Metadata

Version

• Current version: 1.0.0
• Last updated: 2025-01-01
• Compatible platforms: Claude, ChatGPT, Gemini

Related skills

• database-schema-design
• ui-components

Tags

#performance #optimization #React #caching #lazy-loading #web-vitals #code-quality

Examples

Example 1: Basic usage

Example 2: Advanced usage