Next.js & React Performance Expert

From Vercel Engineering - 57 optimization rules prioritized by impact Philosophy: Eliminate waterfalls first, optimize bundles second, then micro-optimize.

🎯 Selective Reading Rule (MANDATORY)

Read ONLY sections relevant to your task! Check the content map below and load what you need.

🔴 For performance reviews: Start with CRITICAL sections (1-2), then move to HIGH/MEDIUM.

📑 Content Map

File Impact Rules When to Read

1-async-eliminating-waterfalls.md

🔴 CRITICAL 5 rules Slow page loads, sequential API calls, data fetching waterfalls

2-bundle-bundle-size-optimization.md

🔴 CRITICAL 5 rules Large bundle size, slow Time to Interactive, First Load issues

3-server-server-side-performance.md

🟠 HIGH 7 rules Slow SSR, API route optimization, server-side waterfalls

4-client-client-side-data-fetching.md

🟡 MEDIUM-HIGH 4 rules Client data management, SWR patterns, deduplication

5-rerender-re-render-optimization.md

🟡 MEDIUM 12 rules Excessive re-renders, React performance, memoization

6-rendering-rendering-performance.md

🟡 MEDIUM 9 rules Rendering bottlenecks, virtualization, image optimization

7-js-javascript-performance.md

⚪ LOW-MEDIUM 12 rules Micro-optimizations, caching, loop performance

8-advanced-advanced-patterns.md

🔵 VARIABLE 3 rules Advanced React patterns, useLatest, init-once

Total: 57 rules across 8 categories

🚀 Quick Decision Tree

What's your performance issue?

🐌 Slow page loads / Long Time to Interactive → Read Section 1: Eliminating Waterfalls → Read Section 2: Bundle Size Optimization

📦 Large bundle size (> 200KB) → Read Section 2: Bundle Size Optimization → Check: Dynamic imports, barrel imports, tree-shaking

🖥️ Slow Server-Side Rendering → Read Section 3: Server-Side Performance → Check: Parallel data fetching, streaming

🔄 Too many re-renders / UI lag → Read Section 5: Re-render Optimization → Check: React.memo, useMemo, useCallback

🎨 Rendering performance issues → Read Section 6: Rendering Performance → Check: Virtualization, layout thrashing

🌐 Client-side data fetching problems → Read Section 4: Client-Side Data Fetching → Check: SWR deduplication, localStorage

✨ Need advanced patterns → Read Section 8: Advanced Patterns

📊 Impact Priority Guide

Use this order when doing comprehensive optimization:

1️⃣ CRITICAL (Biggest Gains - Do First): ├─ Section 1: Eliminating Waterfalls │ └─ Each waterfall adds full network latency (100-500ms+) └─ Section 2: Bundle Size Optimization └─ Affects Time to Interactive and Largest Contentful Paint

2️⃣ HIGH (Significant Impact - Do Second): └─ Section 3: Server-Side Performance └─ Eliminates server-side waterfalls, faster response times

3️⃣ MEDIUM (Moderate Gains - Do Third): ├─ Section 4: Client-Side Data Fetching ├─ Section 5: Re-render Optimization └─ Section 6: Rendering Performance

4️⃣ LOW (Polish - Do Last): ├─ Section 7: JavaScript Performance └─ Section 8: Advanced Patterns

🔗 Related Skills

Need Skill

API design patterns @[skills/api-patterns]

Database optimization @[skills/database-design]

Testing strategies @[skills/testing-patterns]

UI/UX design principles @[skills/frontend-design]

TypeScript patterns @[skills/typescript-expert]

Deployment & DevOps @[skills/deployment-procedures]

✅ Performance Review Checklist

Before shipping to production:

Critical (Must Fix):

• No sequential data fetching (waterfalls eliminated)

• Bundle size < 200KB for main bundle

• No barrel imports in app code

• Dynamic imports used for large components

• Parallel data fetching where possible

High Priority:

• Server components used where appropriate

• API routes optimized (no N+1 queries)

• Suspense boundaries for data fetching

• Static generation used where possible

Medium Priority:

• Expensive computations memoized

• List rendering virtualized (if > 100 items)

• Images optimized with next/image

• No unnecessary re-renders

Low Priority (Polish):

• Hot path loops optimized

• RegExp patterns hoisted

• Property access cached in loops

❌ Anti-Patterns (Common Mistakes)

DON'T:

• ❌ Use sequential await for independent operations

• ❌ Import entire libraries when you need one function

• ❌ Use barrel exports (index.ts re-exports) in app code

• ❌ Skip dynamic imports for large components/libraries

• ❌ Fetch data in useEffect without deduplication

• ❌ Forget to memoize expensive computations

• ❌ Use client components when server components work

DO:

• ✅ Fetch data in parallel with Promise.all()

• ✅ Use dynamic imports: const Comp = dynamic(() => import('./Heavy'))

• ✅ Import directly: import { specific } from 'library/specific'

• ✅ Use Suspense boundaries for better UX

• ✅ Leverage React Server Components

• ✅ Measure performance before optimizing

• ✅ Use Next.js built-in optimizations (next/image, next/font)

🎯 How to Use This Skill

For New Features:

• Check Section 1 & 2 while building (prevent waterfalls, keep bundle small)

• Use server components by default (Section 3)

• Apply memoization for expensive operations (Section 5)

For Performance Reviews:

• Start with Section 1 (waterfalls = biggest impact)

• Then Section 2 (bundle size)

• Then Section 3 (server-side)

• Finally other sections as needed

For Debugging Slow Performance:

• Identify the symptom (slow load, lag, etc.)

• Use Quick Decision Tree above

• Read relevant section

• Apply fixes in priority order

📚 Learning Path

Beginner (Focus on Critical): → Section 1: Eliminating Waterfalls → Section 2: Bundle Size Optimization

Intermediate (Add High Priority): → Section 3: Server-Side Performance → Section 5: Re-render Optimization

Advanced (Full Optimization): → All sections + Section 8: Advanced Patterns

🔍 Validation Script

Script Purpose Command

scripts/react_performance_checker.py

Automated performance audit python scripts/react_performance_checker.py <project_path>

📖 Section Details

Section 1: Eliminating Waterfalls (CRITICAL)

Impact: Each waterfall adds 100-500ms+ latency Key Concepts: Parallel fetching, Promise.all(), Suspense boundaries, preloading

Section 2: Bundle Size Optimization (CRITICAL)

Impact: Directly affects Time to Interactive, Largest Contentful Paint Key Concepts: Dynamic imports, tree-shaking, barrel import avoidance

Section 3: Server-Side Performance (HIGH)

Impact: Faster server responses, better SEO Key Concepts: Parallel server fetching, streaming, API route optimization

Section 4: Client-Side Data Fetching (MEDIUM-HIGH)

Impact: Reduces redundant requests, better UX Key Concepts: SWR deduplication, localStorage caching, event listeners

Section 5: Re-render Optimization (MEDIUM)

Impact: Smoother UI, less wasted computation Key Concepts: React.memo, useMemo, useCallback, component structure

Section 6: Rendering Performance (MEDIUM)

Impact: Better rendering efficiency Key Concepts: Virtualization, image optimization, layout thrashing

Section 7: JavaScript Performance (LOW-MEDIUM)

Impact: Incremental improvements in hot paths Key Concepts: Loop optimization, caching, RegExp hoisting

Section 8: Advanced Patterns (VARIABLE)

Impact: Specific use cases Key Concepts: useLatest hook, init-once patterns, event handler refs

🎓 Best Practices Summary

Golden Rules:

• Measure first - Use React DevTools Profiler, Chrome DevTools

• Biggest impact first - Waterfalls → Bundle → Server → Micro

• Don't over-optimize - Focus on real bottlenecks

• Use platform features - Next.js has optimizations built-in

• Think about users - Real-world conditions matter

Performance Mindset:

• Every await in sequence = potential waterfall

• Every import = potential bundle bloat

• Every re-render = wasted computation (if unnecessary)

• Server components = less JavaScript to ship

• Measure, don't guess

Source: Vercel Engineering Date: January 2026 Version: 1.0.0 Total Rules: 57 across 8 categories