debug-performance

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name: debug-performance description: Diagnoses and fixes performance problems in web apps: slow renders, memory leaks, large bundles, waterfall requests, and poor Core Web Vitals. Use when the user says "my app is slow", "too many re-renders", "memory leak", "LCP is bad", "bundle is too large", "this feels laggy", "optimize this component", or shares performance profiler output or Lighthouse results.

Debug Performance

Diagnoses web application performance problems across four layers: rendering, network, bundle, and memory. Identifies the root cause, not just symptoms, and provides concrete fixes with measurable impact.

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Required Details

Detail	Example
Symptom	"Laggy UI", "slow LCP", "memory grows over time"
Framework	Next.js / React / vanilla JS
Code or profiler output	Component, network waterfall, Lighthouse report
Where it's slow	Initial load / interaction / specific page

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Instructions

Step 1 — Identify the layer

Symptom	Likely layer
Laggy UI, janky interactions	Rendering — re-renders or layout thrash
Slow initial page load	Network waterfall or bundle size
Bad LCP / CLS / INP score	Core Web Vitals — specific root causes per metric
Memory grows without release	Memory leak
Slow after many interactions	Memory leak or accumulating state
Slow API responses affecting UI	Network — waterfall or missing cache

Step 2 — Diagnose by layer

Rendering (React)

// Diagnose: add to component to count renders
let renderCount = 0;
console.log('renders:', ++renderCount);

// Fix unnecessary re-renders
// ❌ Object created on every render — breaks memo
<Child config={{ theme: 'dark' }} />

// ✅ Stable reference
const config = useMemo(() => ({ theme: 'dark' }), []);
<Child config={config} />

Common causes:

• Props with inline objects/arrays/functions (onClick={() => ...})
• Missing React.memo on expensive pure components
• State too high — move state down to the component that needs it
• useEffect with missing or wrong dependencies causing loops

Bundle size

// Diagnose: add to next.config.ts
import { BundleAnalyzerPlugin } from 'webpack-bundle-analyzer';

// Fix: dynamic import for heavy components
const HeavyChart = dynamic(() => import('@/components/HeavyChart'), {
  loading: () => <Skeleton />,
  ssr: false,
});

// Fix: import only what you need
// ❌
import _ from 'lodash';
// ✅
import debounce from 'lodash/debounce';

Core Web Vitals

Metric	Root cause	Fix
LCP (slow largest paint)	Unoptimized hero image	<Image priority />, preload, WebP/AVIF
CLS (layout shift)	Images without dimensions	Always set width + height on images
INP (slow interaction)	Heavy JS on main thread	startTransition, useDeferredValue, Web Workers

Memory leaks

// ❌ Event listener never removed
useEffect(() => {
  window.addEventListener('resize', handler);
}, []); // missing cleanup

// ✅ Cleanup on unmount
useEffect(() => {
  window.addEventListener('resize', handler);
  return () => window.removeEventListener('resize', handler);
}, []);

Common memory leaks:

• Event listeners without cleanup in useEffect
• setInterval / setTimeout without clearInterval / clearTimeout
• Subscriptions (WebSocket, RxJS, EventEmitter) not unsubscribed
• Closures holding references to large objects

Network waterfall

// ❌ Sequential fetches — waterfall
const user = await getUser(id);
const posts = await getPosts(user.id); // waits for user first

// ✅ Parallel fetches
const [user, posts] = await Promise.all([
  getUser(id),
  getPosts(id), // both start at the same time
]);

Step 3 — Verify the fix

For each fix, provide a way to confirm it worked:

• Re-renders: React DevTools Profiler → check render count dropped
• Bundle: next build output → check chunk sizes
• LCP/CLS/INP: Lighthouse re-run → check score improvement
• Memory: Chrome DevTools Memory tab → heap snapshot before/after

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Non-Negotiable Acceptance Criteria

• Root cause identified — not just the symptom
• Each fix includes code showing before (❌) and after (✅)
• Each fix includes how to verify it worked
• Fixes are ordered by impact — highest impact first
• If multiple layers are affected, each is addressed separately

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Output Format

## Performance Diagnosis

### Root Cause
[What is actually causing the problem — be specific]

### Layer: [Rendering / Bundle / Network / Memory]

**Problem**
[explanation + problematic code]

**Fix**
[corrected code]

**How to verify**
[tool and what to look for]

[repeat for each layer affected]

### Expected Impact
[rough estimate: "should reduce re-renders by ~80%", "LCP improvement of ~1s"]