# React Component Performance
## Overview
Identify render hotspots, isolate expensive updates, and apply targeted optimizations without changing UI behavior.
## When to Use
- When the user asks to profile or improve a slow React component.
- When you need to reduce re-renders, list lag, or expensive render work in React UI.
## Workflow
1. Reproduce or describe the slowdown.
2. Identify what triggers re-renders (state updates, props churn, effects).
3. Isolate fast-changing state from heavy subtrees.
4. Stabilize props and handlers; memoize where it pays off.
5. Reduce expensive work (computation, DOM size, list length).
6. **Validate**: open React DevTools Profiler → record the interaction → inspect the Flamegraph for components rendering longer than ~16 ms → compare against a pre-optimization baseline recording.
## Checklist
- Measure: use React DevTools Profiler or log renders; capture baseline.
- Find churn: identify state updated on a timer, scroll, input, or animation.
- Split: move ticking state into a child; keep heavy lists static.
- Memoize: wrap leaf rows with `memo` only when props are stable.
- Stabilize props: use `useCallback`/`useMemo` for handlers and derived values.
- Avoid derived work in render: precompute, or compute inside memoized helpers.
- Control list size: window/virtualize long lists; avoid rendering hidden items.
- Keys: ensure stable keys; avoid index when order can change.
- Effects: verify dependency arrays; avoid effects that re-run on every render.
- Style/layout: watch for expensive layout thrash or large Markdown/diff renders.
## Optimization Patterns
### Isolate ticking state
Move a timer or animation counter into a child so the parent list never re-renders on each tick.
```tsx
// ❌ Before – entire parent (and list) re-renders every second
function Dashboard({ items }: { items: Item[] }) {
const [tick, setTick] = useState(0);
useEffect(() => {
const id = setInterval(() => setTick(t => t + 1), 1000);
return () => clearInterval(id);
}, []);
return (
{/* re-renders every second */}
re-renders; list is untouched
function Clock() {
const [tick, setTick] = useState(0);
useEffect(() => {
const id = setInterval(() => setTick(t => t + 1), 1000);
return () => clearInterval(id);
}, []);
return
{tick}s;
}
function Dashboard({ items }: { items: Item[] }) {
return (
console.log(id); // new ref each render
return items.map(item => );
}
// ✅ After – stable handler; Row only re-renders when its own item changes
const Row = memo(({ item, onClick }: RowProps) => (
onClick(item.id)}>{item.name}
));
function List({ items }: { items: Item[] }) {
const handleClick = useCallback((id: string) => console.log(id), []);
return items.map(item => );
}
```
### Prefer derived data outside render
```tsx
// ❌ Before – recomputes on every render
function Summary({ orders }: { orders: Order[] }) {
const total = orders.reduce((sum, o) => sum + o.amount, 0); // runs every render
return
Total: {total}
;
}
// ✅ After – recomputes only when orders changes
function Summary({ orders }: { orders: Order[] }) {
const total = useMemo(() => orders.reduce((sum, o) => sum + o.amount, 0), [orders]);
return
Total: {total}
;
}
```
### Additional patterns
- **Split rows**: extract list rows into memoized components with narrow props.
- **Defer heavy rendering**: lazy-render or collapse expensive content until expanded.
## Profiling Validation Steps
1. Open **React DevTools → Profiler** tab.
2. Click **Record**, perform the slow interaction, then **Stop**.
3. Switch to **Flamegraph** view; any bar labeled with a component and time > ~16 ms is a candidate.
4. Use **Ranked chart** to sort by self render time and target the top offenders.
5. Apply one optimization at a time, re-record, and compare render counts and durations against the baseline.
## Example Reference
Load `references/examples.md` when the user wants a concrete refactor example.