When will you use this?

Apply when users complain the app feels slow, or Lighthouse shows red scores.

• Slow admin tables with thousands of rows need memo, lazy load, or virtualization.
• Lighthouse scores matter for public sites — clients notice slow pages.

Real-world: profile React DevTools

Order page felt slow — Profiler shows Parent re-rendering 200 Row components. Fix: memo Row + stable callback; INP drops 40%.

Production-style code

// React DevTools Profiler steps:
// 1. Record interaction (apply filter)
// 2. Find components with longest render duration
// 3. Check "why did this render?"

const OrderRow = React.memo(function OrderRow({ order }) {
  return <tr><td>{order.id}</td><td>{order.total}</td></tr>;
});

// Before fix: OrderTable 180ms render
// After memo + useCallback: OrderTable 12ms render

What happens in production: Profile before guessing — senior engineers always measure render cost on real data sets.

Lesson example (start here)

Copy this smaller example first. Once it works, compare it with the real-world code above.

// 1. Profiler → slow ProductGrid
// 2. memo row component
// 3. virtualize list
// 4. lazy load chart library
// 5. Re-profile — confirm fix

Line-by-line walkthrough

How it works (big picture)

• Always measure before and after.
• One fix at a time so you know what helped.

Do this on your computer

1. Record baseline Lighthouse score.
2. Profile the slowest interaction.
3. Apply one fix and remeasure.
4. Read the real-world section and name which part of the app uses this topic.
5. Run the example locally and confirm the same behavior in the browser.
6. Change one value in the example (text, initial state, or URL) and predict what will happen before you save.

Experiments — try changing this

• Change text or labels in the example and save — watch the browser update.
• Break the code on purpose (remove a bracket), read the error message, then fix it.
• Open React DevTools (browser extension) while running Frontend Performance Tuning and inspect component props/state.

Remember

Measure → fix → measure. Target biggest bottleneck first. Combine bundle and render optimizations.

Common questions