JustineDevs/nextjs-performance

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):

'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):

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

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

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

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

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

// 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

'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

'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

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

# Run analysis
ANALYZE=true npm run build

// 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

// ❌ 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
• Core Web Vitals
• React Server Components