# Performance Optimization

> Performance optimization patterns for Next.js applications. Covers bundle analysis, React rendering optimization, database query optimization, Core Web Vitals, image optimization, and caching strategies.

- Skill: `janszewczyk/performance-optimization` (Agent Skill, multi-file: 5 files)
- Install (CLI): `npx skillmds@latest add janszewczyk/performance-optimization`
- Raw SKILL.md: https://api.skillmd.com/api/skills/janszewczyk/performance-optimization/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: JanSzewczyk (https://skillmd.com/u/janszewczyk)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/janszewczyk/performance-optimization

---


# Performance Optimization Skill

Performance optimization patterns for Next.js applications.

> **Reference Files:**
>
> - [bundle-analysis.md](./references/bundle-analysis.md) - Bundle size optimization
> - [react-rendering.md](./references/react-rendering.md) - React performance patterns
> - [database-optimization.md](./references/database-optimization.md) - Query optimization
> - [examples.md](./references/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

```bash
# Analyze bundle
npm run analyze

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

**Quick Wins:**

```typescript
// 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

```typescript
// 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

```typescript
// 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

```bash
# 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