name	performance-optimization
description	Use when optimizing code for performance, reducing bundle size, improving load times, or fixing performance bottlenecks. Emphasizes measurement-driven optimization.
allowed-tools	Read, Write, Edit, Bash, Grep, Glob

Performance Optimization Skill

Systematic approach to identifying and fixing performance issues.

Core Principle

Measure, don't guess. Optimization without data is guesswork.

The Cardinal Rule

❌ NEVER optimize without measuring first

Why: Premature optimization wastes time on non-issues while missing real problems.

Exception: Obvious O(n²) algorithms when O(n) alternatives exist.

Optimization Process

1. Measure Current State (Baseline)

Before touching any code, establish metrics:

Frontend Performance:

# Chrome DevTools Performance tab
# Lighthouse audit
npm run build && du -sh dist/  # Bundle size

Backend Performance:

# Add timing logs
start = Time.now
result = expensive_operation()
elapsed = Time.now - start
Logger.info("Operation took #{elapsed}ms")

Database:

# PostgreSQL
EXPLAIN ANALYZE SELECT ...;

# Check query time in logs
grep "SELECT" logs/production.log | grep "Duration:"

Metrics to capture:

Load time / response time
Time to interactive
Bundle size
Memory usage
Query duration
Render time

2. Profile to Find Bottlenecks

Don't guess where the problem is - profile:

Browser Profiling:

Chrome DevTools > Performance tab
Record interaction
Look for long tasks (> 50ms)
Check for layout thrashing

Server Profiling:

# Add detailed timing
defmodule Profiler do
  def measure(label, func) do
    start = System.monotonic_time(:millisecond)
    result = func.()
    elapsed = System.monotonic_time(:millisecond) - start
    Logger.info("#{label}: #{elapsed}ms")
    result
  end
end

# Use it
Profiler.measure("Database query", fn ->
  Repo.all(User)
end)

React Profiling:

# React DevTools Profiler
# Look for:
# - Unnecessary re-renders
# - Slow components (> 16ms for 60fps)
# - Large component trees

3. Identify Root Cause

Common performance issues:

Frontend:

Large bundle size (lazy load, code split)
Unnecessary re-renders (memoization)
Blocking JavaScript (defer, async)
Unoptimized images (WebP, lazy loading)
Too many network requests (bundle, cache)
Memory leaks (cleanup useEffect)

Backend:

N+1 queries (preload associations)
Missing database indexes
Expensive computations in loops
Synchronous external API calls
Large JSON responses
Inefficient algorithms

Database:

Missing indexes
Inefficient query structure
Too many joins
Fetching unnecessary columns
No query result caching

4. Apply Targeted Optimization

One change at a time - Measure impact of each change

Frontend Optimizations

Bundle Size Reduction:

// Before: Import entire library
import _ from 'lodash'

// After: Import only what's needed
import debounce from 'lodash/debounce'

// Or: Use native alternatives
const unique = [...new Set(array)]  // Instead of _.uniq(array)

React Performance:

// Before: Re-renders on every parent render
function ChildComponent({ items }) {
  return <div>{items.map(...)}</div>
}

// After: Only re-render when items change
const ChildComponent = React.memo(function ChildComponent({ items }) {
  return <div>{items.map(...)}</div>
}, (prev, next) => prev.items === next.items)

// Before: Recreates function every render
function Parent() {
  const handleClick = () => { ... }
  return <Child onClick={handleClick} />
}

// After: Stable function reference
function Parent() {
  const handleClick = useCallback(() => { ... }, [])
  return <Child onClick={handleClick} />
}

Code Splitting:

// Before: All in main bundle
import HeavyComponent from './HeavyComponent'

// After: Lazy load when needed
const HeavyComponent = React.lazy(() => import('./HeavyComponent'))

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <HeavyComponent />
    </Suspense>
  )
}

Image Optimization:

// Before: Full-size image
<img src="/hero.jpg" />

// After: Responsive, lazy-loaded
<img
  src="/hero-800w.webp"
  srcSet="/hero-400w.webp 400w, /hero-800w.webp 800w"
  loading="lazy"
  alt="Hero image"
/>

Backend Optimizations

N+1 Query Fix:

# Before: N+1 queries (1 for users + N for posts)
users = Repo.all(User)
Enum.map(users, fn user ->
  posts = Repo.all(from p in Post, where: p.user_id == ^user.id)
  {user, posts}
end)

# After: 2 queries total
users = Repo.all(User) |> Repo.preload(:posts)
Enum.map(users, fn user -> {user, user.posts} end)

Database Indexing:

-- Before: Slow query
SELECT * FROM users WHERE email = 'user@example.com';
-- Seq Scan (5000ms)

-- After: Add index
CREATE INDEX idx_users_email ON users(email);
-- Index Scan (2ms)

Caching:

# Before: Expensive calculation every request
def get_popular_posts do
  # Complex aggregation query (500ms)
  Repo.all(from p in Post, ...)
end

# After: Cache for 5 minutes
def get_popular_posts do
  Cachex.fetch(:app_cache, "popular_posts", fn ->
    result = Repo.all(from p in Post, ...)
    {:commit, result, ttl: :timer.minutes(5)}
  end)
end

Batch Processing:

# Before: Process one at a time
Enum.each(user_ids, fn id ->
  user = Repo.get(User, id)
  send_email(user)
end)

# After: Batch fetch
users = Repo.all(from u in User, where: u.id in ^user_ids)
Enum.each(users, &send_email/1)

Algorithm Optimization

Reduce Complexity:

// Before: O(n²) - nested loops
function findDuplicates(arr: number[]): number[] {
  const duplicates = []
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      if (arr[i] === arr[j] && !duplicates.includes(arr[i])) {
        duplicates.push(arr[i])
      }
    }
  }
  return duplicates
}

// After: O(n) - single pass with Set
function findDuplicates(arr: number[]): number[] {
  const seen = new Set<number>()
  const duplicates = new Set<number>()

  for (const num of arr) {
    if (seen.has(num)) {
      duplicates.add(num)
    }
    seen.add(num)
  }

  return Array.from(duplicates)
}

5. Measure Impact (Proof of Work)

ALWAYS measure after optimization:

## Optimization: [What was changed]

### Before
- Load time: 3.2s
- Bundle size: 850KB
- Time to interactive: 4.1s

### Changes
- Lazy loaded HeavyComponent
- Switched to lodash-es for tree shaking
- Added React.memo to ProductList

### After
- Load time: 1.8s (-44%)
- Bundle size: 520KB (-39%)
- Time to interactive: 2.3s (-44%)

### Evidence
```bash
# Before
$ npm run build
dist/main.js   850.2 KB

# After
$ npm run build
dist/main.js   520.8 KB


**Use proof-of-work skill to document evidence**

### 6. Verify Correctness

**Tests must still pass:**

```bash
# Run full test suite
npm test        # Frontend
mix test        # Backend

# Manual verification
# - Feature still works
# - Edge cases handled
# - No new bugs introduced

Common Optimization Targets

Frontend Checklist

Bundle size < 200KB (gzipped)
First Contentful Paint < 1.5s
Time to Interactive < 3s
No layout shift (CLS < 0.1)
Images optimized (WebP, lazy loading)
Code split by route
Unused code removed (tree shaking)
CSS critical path optimized

Backend Checklist

API response time < 200ms (p95)
Database queries optimized (EXPLAIN ANALYZE)
No N+1 queries
Appropriate indexes exist
Expensive operations cached
Background jobs for slow tasks
Connection pooling configured
Pagination for large datasets

Database Checklist

Indexes on frequently queried columns
Query execution plan reviewed
No full table scans
Appropriate use of LIMIT
Joins optimized (smallest table first)
Statistics up to date (ANALYZE)

Optimization Patterns

Lazy Loading Pattern

// Route-based code splitting
const routes = [
  {
    path: '/admin',
    component: lazy(() => import('./pages/Admin'))
  },
  {
    path: '/dashboard',
    component: lazy(() => import('./pages/Dashboard'))
  }
]

Memoization Pattern

// Expensive calculation
const ExpensiveComponent = ({ data }) => {
  // Only recalculate when data changes
  const processedData = useMemo(() => {
    return data.map(item => expensiveTransform(item))
  }, [data])

  return <div>{processedData.map(...)}</div>
}

Database Query Optimization Pattern

# Instead of multiple queries
users = Repo.all(User)
posts = Repo.all(Post)
comments = Repo.all(Comment)

# Use join and preload
users =
  User
  |> join(:left, [u], p in assoc(u, :posts))
  |> join(:left, [u, p], c in assoc(p, :comments))
  |> preload([u, p, c], [posts: {p, comments: c}])
  |> Repo.all()

Anti-Patterns

❌ Optimizing the Wrong Thing

BAD: Spending hours optimizing function that runs once
GOOD: Optimize the function that runs 10,000 times per page load

Always profile first to find real bottlenecks

❌ Premature Optimization

BAD: "This might be slow, let me optimize it"
GOOD: "This IS slow (measured 500ms), let me optimize it"

❌ Micro-optimizations

BAD: Replacing `.map()` with `for` loop to save 1ms
GOOD: Reducing bundle size by 200KB to save 1000ms

Focus on high-impact optimizations

❌ Breaking Functionality for Performance

BAD: Remove feature to make it faster
GOOD: Keep feature, make implementation faster

Performance should not come at cost of correctness

❌ Optimizing Without Evidence

BAD: "I think this will be faster" [changes code]
GOOD: "Profiler shows this takes 80% of time" [measures, optimizes, measures again]

Trade-offs to Consider

Performance vs Readability:

// More readable
const result = items
  .filter(item => item.active)
  .map(item => item.name)

// Faster (one loop instead of two)
const result = []
for (const item of items) {
  if (item.active) {
    result.push(item.name)
  }
}

Question: Is the perf gain worth the readability loss? Profile first.

Performance vs Maintainability:

Caching adds complexity
Memoization adds memory overhead
Code splitting adds bundle management

Always document the trade-off made

Tools & Commands