React Component Performance

---
name: react-component-performance
description: Diagnose slow React components and suggest targeted performance fixes.
---

# 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 (
    <>
      <Clock tick={tick} />
      <ExpensiveList items={items} /> {/* re-renders every second */}
    </>
  );
}

// ✅ After – only <Clock> 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 <span>{tick}s</span>;
}

function Dashboard({ items }: { items: Item[] }) {
  return (
    <>
      <Clock />
      <ExpensiveList items={items} />
    </>
  );
}
```

### Stabilize callbacks with `useCallback` + `memo`

```tsx
// ❌ Before – new handler reference on every render busts Row memo
function List({ items }: { items: Item[] }) {
  const handleClick = (id: string) => console.log(id); // new ref each render
  return items.map(item => <Row key={item.id} item={item} onClick={handleClick} />);
}

// ✅ After – stable handler; Row only re-renders when its own item changes
const Row = memo(({ item, onClick }: RowProps) => (
  <li onClick={() => onClick(item.id)}>{item.name}</li>
));

function List({ items }: { items: Item[] }) {
  const handleClick = useCallback((id: string) => console.log(id), []);
  return items.map(item => <Row key={item.id} item={item} onClick={handleClick} />);
}
```

### 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 <p>Total: {total}</p>;
}

// ✅ After – recomputes only when orders changes
function Summary({ orders }: { orders: Order[] }) {
  const total = useMemo(() => orders.reduce((sum, o) => sum + o.amount, 0), [orders]);
  return <p>Total: {total}</p>;
}
```

### 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.

## Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.