nextjs-performance — Claude Code skill: what it does, install

---
name: nextjs-performance
description: Expert Next.js performance optimization skill covering Core Web Vitals, image/font optimization, caching strategies, streaming, bundle optimization, and Server Components best practices. Use when optimizing Next.js applications for Core Web Vitals (LCP, INP, CLS), implementing next/image and next/font, configuring caching with unstable_cache and revalidateTag, converting Client Components to Server Components, implementing Suspense streaming, or analyzing and reducing bundle size. Supports Next.js 16 + React 19 patterns.
allowed-tools: Read, Write, Edit, Bash, Glob, Grep
---

# Next.js Performance Optimization

Expert guidance for optimizing Next.js applications with focus on Core Web Vitals, modern patterns, and best practices.

## Overview

This skill provides comprehensive guidance for optimizing Next.js applications. It covers Core Web Vitals optimization (LCP, INP, CLS), modern React patterns, Server Components, caching strategies, and bundle optimization techniques. Designed for developers already familiar with React/Next.js who want to implement production-grade optimizations.

## When to Use

Use this skill when working on Next.js applications and need to:

- Optimize Core Web Vitals (LCP, INP, CLS) for better performance and SEO
- Implement image optimization with `next/image` for faster loading
- Configure font optimization with `next/font` to eliminate layout shift
- Set up caching strategies using `unstable_cache`, `revalidateTag`, or ISR
- Convert Client Components to Server Components for reduced bundle size
- Implement Suspense streaming for progressive page loading
- Analyze and reduce bundle size with code splitting and dynamic imports
- Configure metadata and SEO for better search engine visibility
- Optimize API route handlers for better performance
- Apply Next.js 16 and React 19 modern patterns

### Coverage Areas

- **Core Web Vitals optimization** (LCP, INP, CLS)
- **Image optimization** with `next/image`
- **Font optimization** with `next/font`
- **Caching strategies** (`unstable_cache`, `revalidateTag`, ISR)
- **Server Components** patterns and Client-to-Server conversion
- **Streaming and Suspense** for progressive loading
- **Bundle optimization** and code splitting
- **Metadata and SEO** configuration
- **Route handlers** optimization
- **Next.js 16 + React 19** patterns

## Instructions

### Before Starting

1. **Analyze current performance** with Lighthouse
2. **Identify bottlenecks** - check Core Web Vitals in Chrome DevTools or PageSpeed Insights
3. **Determine optimization priority**:
   - LCP issues → Focus on images, fonts
   - INP issues → Reduce JS, use Server Components
   - CLS issues → Add dimensions, use next/font

### How to Use This Skill

1. **Load relevant reference files** based on the area you're optimizing:
   - Image issues → `references/image-optimization.md`
   - Font/layout shift → `references/font-optimization.md`
   - Caching → `references/caching-strategies.md`
   - Component architecture → `references/server-components.md`

2. **Follow the quick patterns** for common optimizations
3. **Apply before/after conversions** to improve existing code
4. **Verify improvements** with Lighthouse after changes

### Core Principles

1. **Prefer Server Components** - Only use 'use client' when necessary (browser APIs, interactivity)
2. **Load components as low as possible** - Keep Client Components at leaf nodes
3. **Use Suspense boundaries** - Enable streaming and progressive loading
4. **Cache appropriately** - Use tags for granular revalidation
5. **Measure before/after** - Always verify improvements with real metrics

## Examples

### Example 1: Convert Client Component to Server Component

**BEFORE (Client Component with useEffect):**
```tsx
'use client'
import { useEffect, useState } from 'react'

export default function ProductList() {
  const [products, setProducts] = useState([])

  useEffect(() => {
    fetch('/api/products').then(r => r.json()).then(setProducts)
  }, [])

  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>
}
```

**AFTER (Server Component with direct data access):**
```tsx
import { db } from '@/lib/db'

export default async function ProductList() {
  const products = await db.product.findMany()
  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>
}
```

### Example 2: Optimize Images for LCP

```tsx
import Image from 'next/image'

export function Hero() {
  return (
    <div className="relative w-full h-[600px]">
      <Image
        src="/hero.jpg"
        alt="Hero"
        fill
        priority          // Disable lazy loading for LCP
        sizes="100vw"
        className="object-cover"
      />
    </div>
  )
}
```

### Example 3: Implement Caching Strategy

```tsx
import { unstable_cache, revalidateTag } from 'next/cache'

// Cached data function
const getProducts = unstable_cache(
  async () => db.product.findMany(),
  ['products'],
  { revalidate: 3600, tags: ['products'] }
)

// Revalidate on mutation
export async function createProduct(data: FormData) {
  'use server'
  await db.product.create({ data })
  revalidateTag('products')
}
```

### Example 4: Setup Optimized Fonts

```tsx
import { Inter } from 'next/font/google'

const inter = Inter({
  subsets: ['latin'],
  display: 'swap',
  variable: '--font-inter',
})

export default function RootLayout({ children }) {
  return (
    <html lang="en" className={inter.variable}>
      <body className={`${inter.className} antialiased`}>
        {children}
      </body>
    </html>
  )
}
```

### Example 5: Implement Suspense Streaming

```tsx
import { Suspense } from 'react'

export default function Page() {
  return (
    <>
      <header>Static content (immediate)</header>

      <Suspense fallback={<ProductSkeleton />}>
        <ProductList />  {/* Streamed when ready */}
      </Suspense>

      <Suspense fallback={<ReviewsSkeleton />}>
        <Reviews />      {/* Independent streaming */}
      </Suspense>
    </>
  )
}
```

## Reference Documentation

Load these references when working on specific areas:

| Topic | Reference File |
|-------|----------------|
| Core Web Vitals | `references/core-web-vitals.md` |
| Image Optimization | `references/image-optimization.md` |
| Font Optimization | `references/font-optimization.md` |
| Caching Strategies | `references/caching-strategies.md` |
| Server Components | `references/server-components.md` |
| Streaming/Suspense | `references/streaming-suspense.md` |
| Bundle Optimization | `references/bundle-optimization.md` |
| Metadata/SEO | `references/metadata-seo.md` |
| API Routes | `references/api-routes.md` |
| Next.js 16 Patterns | `references/nextjs-16-patterns.md` |

## Common Conversions

| From | To | Benefit |
|------|-----|---------|
| `useEffect` + fetch | Direct async in Server Component | -70% JS, faster TTFB |
| `useState` for data | Server Component with direct DB access | Simpler code, no hydration |
| Client-side fetch | `unstable_cache` or ISR | Faster repeated loads |
| `img` tag | `next/image` | Optimized formats, lazy loading |
| CSS font import | `next/font` | Zero CLS, automatic optimization |
| Static import of heavy component | `dynamic()` | Reduced initial bundle |

## Best Practices

### Images
- Use `next/image` for all images
- Add `priority` to LCP images only
- Provide `width` and `height` or `fill` with sizes
- Use `placeholder="blur"` for better UX
- Configure remotePatterns in next.config.js

### Fonts
- Use `next/font` instead of CSS imports
- Specify `subsets` to reduce size
- Use `display: 'swap'` for immediate text render
- Create CSS variable with `variable` option
- Configure Tailwind to use CSS variables

### Caching
- Cache expensive queries with `unstable_cache`
- Use meaningful cache tags for granular control
- Implement on-demand revalidation for dynamic content
- Set TTL based on data change frequency
- Use revalidatePath for route-level invalidation

### Components
- Convert Client Components to Server Components where possible
- Keep Client Components at the leaf level
- Use Suspense boundaries for progressive loading
- Implement proper loading states
- Use dynamic() for heavy components below the fold

### Bundle
- Lazy load heavy components with `dynamic()`
- Use named exports for better tree shaking
- Analyze bundle regularly with `@`next/bundle-analyzer
- Prefer ESM packages over CommonJS
- Use modularizeImports for large libraries

## Constraints and Warnings

### Server Components Limitations
- Cannot use browser APIs (window, localStorage, document)
- Cannot use React hooks (useState, useEffect, useContext)
- Cannot use event handlers (onClick, onSubmit)
- Cannot use dynamic imports with ssr: false

### Image Optimization Constraints
- `priority` should only be used for above-the-fold images
- External images require configuration in next.config.js
- `width` and `height` are required unless using `fill`
- Animated GIFs are not optimized by default

### Caching Considerations
- Cache tags must be manually invalidated
- Data cache is per-request in development
- Edge runtime has different caching behavior
- Be careful caching user-specific data

### Bundle Size Warnings
- Dynamic imports can impact SEO if critical content
- Tree shaking requires proper ES module usage
- Some libraries cannot be tree shaken (avoid barrel exports)
- Client Components increase bundle size - use sparingly

## Next.js 16 + React 19 Specifics

### Async Params

```tsx
// Next.js 15+ params is a Promise
export default async function Page({
  params,
}: {
  params: Promise<{ slug: string }>
}) {
  const { slug } = await params
  const post = await fetchPost(slug)
  return <article>{post.content}</article>
}
```

### use() Hook for Promises

```tsx
'use client'
import { use, Suspense } from 'react'

function Comments({ promise }: { promise: Promise<Comment[]> }) {
  const comments = use(promise)  // Suspend until resolved
  return <ul>{comments.map(c => <li key={c.id}>{c.text}</li>)}</ul>
}
```

### useOptimistic for UI Updates

```tsx
'use client'
import { useOptimistic } from 'react'

export function TodoList({ todos }: { todos: Todo[] }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo: Todo) => [...state, newTodo]
  )

  async function addTodo(formData: FormData) {
    const text = formData.get('text') as string
    addOptimisticTodo({ id: crypto.randomUUID(), text, completed: false })
    await createTodo(text)
  }

  return (
    <form action={addTodo}>
      <input name="text" />
      {optimisticTodos.map(todo => <div key={todo.id}>{todo.text}</div>)}
    </form>
  )
}
```

## Bundle Analysis

```bash
# Install analyzer
npm install --save-dev @next/bundle-analyzer

# Run analysis
ANALYZE=true npm run build
```

```javascript
// next.config.js
const withBundleAnalyzer = require('@next/bundle-analyzer')({
  enabled: process.env.ANALYZE === 'true',
})

module.exports = withBundleAnalyzer({
  modularizeImports: {
    'lodash': { transform: 'lodash/{{member}}' },
  },
})
```

## Performance Checklist

- [ ] All images use `next/image` with proper dimensions
- [ ] LCP images have `priority` attribute
- [ ] Fonts use `next/font` with subsets
- [ ] Server Components used where possible
- [ ] Client Components at leaf level only
- [ ] Suspense boundaries for data fetching
- [ ] Caching configured for expensive operations
- [ ] Bundle analyzed for duplicates
- [ ] Heavy components lazy loaded
- [ ] Lighthouse score verified before/after

## Common Mistakes

```tsx
// ❌ DON'T: Fetch in useEffect
'use client'
useEffect(() => { fetch('/api/data').then(...) }, [])

// ✅ DO: Fetch directly in Server Component
const data = await fetch('/api/data')

// ❌ DON'T: Forget dimensions on images
<Image src="/photo.jpg" />

// ✅ DO: Always provide dimensions
<Image src="/photo.jpg" width={800} height={600} />

// ❌ DON'T: Use priority on all images
<Image src="/photo1.jpg" priority />
<Image src="/photo2.jpg" priority />

// ✅ DO: Priority only for LCP
<Image src="/hero.jpg" priority />
<Image src="/photo.jpg" loading="lazy" />

// ❌ DON'T: Cache everything with same TTL
{ revalidate: 3600 }

// ✅ DO: Match TTL to data change frequency
{ revalidate: 86400 } // Categories rarely change
{ revalidate: 60 }     // Comments change often
```

## External Resources

- [Next.js Performance Documentation](https://nextjs.org/docs/app/building-your-application/optimizing)
- [Core Web Vitals](https://web.dev/vitals/)
- [React Server Components](https://react.dev/reference/react/server-components)