Performance Optimization Skill

Performance optimization patterns for Next.js applications.

Reference Files:

• bundle-analysis.md - Bundle size optimization
• react-rendering.md - React performance patterns
• database-optimization.md - Query optimization
• examples.md - Practical examples

Performance Philosophy

Principles:

1. Measure first - Never optimize without metrics
2. Target bottlenecks - Focus on the biggest impact areas
3. User-centric - Prioritize perceived performance
4. Iterative - Small, measurable improvements

Quick Reference

Bundle Size

# Analyze bundle
npm run analyze

# Targets
# - Total: < 200KB gzipped (first load)
# - Per-route: < 100KB gzipped

Quick Wins:

// Dynamic imports for heavy components
const Chart = dynamic(() => import("./Chart"), { ssr: false });

// Prefer native implementations
const debounce = (fn: Function, ms: number) => {
  // ✅ Native
  let timeout: NodeJS.Timeout;
  return (...args: unknown[]) => {
    clearTimeout(timeout);
    timeout = setTimeout(() => fn(...args), ms);
  };
};

React Rendering

// With React Compiler - usually automatic optimization
// Manual memoization only for:

// 1. External library callbacks
const stableHandler = useCallback(() => {}, []);

// 2. Context values
const value = useMemo(() => ({ state, dispatch }), [state]);

// 3. Virtualized lists (50+ items)
import { useVirtualizer } from "@tanstack/react-virtual";

Database Queries

// Always use limits
const query = db.collection("items").where("userId", "==", userId).limit(20);

// Parallel fetching
const [users, posts] = await Promise.all([getUsers(), getPosts()]);

Core Web Vitals

Metric	Target	Optimization
LCP	< 2.5s	priority on hero image, preload fonts
INP	< 200ms	startTransition, defer non-critical JS
CLS	< 0.1	Fixed dimensions, skeleton loaders

Performance Targets

Bundle Size Targets

Category	Target (gzipped)
First Load JS	< 100KB
Per-page JS	< 50KB
Total app	< 300KB
Single dependency	< 30KB

Runtime Targets

Metric	Good	Needs Work
Time to Interactive	< 3s	> 5s
First Contentful Paint	< 1.8s	> 3s
Server Response	< 200ms	> 500ms
Database Query	< 100ms	> 500ms

Analysis Commands

# Bundle analysis
npm run analyze

# Build output
npm run build
# Check .next/static/chunks sizes

# Lighthouse (via Chrome DevTools)
# Performance tab → Lighthouse

# React Profiler
# React DevTools → Profiler tab

Decision Tree

Performance Issue?
    │
    ├─ Slow page load?
    │   ├─ Large bundle → Bundle analysis
    │   ├─ Slow API → Database optimization
    │   └─ Render blocking → Code splitting
    │
    ├─ Slow interactions?
    │   ├─ Long lists → Virtualization
    │   ├─ Heavy computation → Web Worker / useMemo
    │   └─ Frequent re-renders → React Profiler
    │
    └─ Layout shifts?
        ├─ Images → Set dimensions
        ├─ Fonts → Font preloading
        └─ Dynamic content → Skeleton loaders

Common Issues & Solutions

Issue	Detection	Solution
Large bundle	> 100KB first load	Dynamic imports, tree shaking
Slow renders	React Profiler > 16ms	Memoization, virtualization
N+1 queries	Multiple sequential DB calls	Batch queries, denormalization
Layout shift	CLS > 0.1	Fixed dimensions, skeletons
Unoptimized images	Large image files	next/image, WebP, responsive

Related Skills

• react-19-compiler - React Compiler optimization guidance
• firebase-firestore - Database query patterns
• structured-logging - Performance logging