react-component-performance

SKILL.md

React Component Performance

Overview

Identify render hotspots, isolate expensive updates, and apply targeted optimizations without changing UI behavior.

Workflow

Reproduce or describe the slowdown.

Identify what triggers re-renders (state updates, props churn, effects).

Isolate fast-changing state from heavy subtrees.

Stabilize props and handlers; memoize where it pays off.

Reduce expensive work (computation, DOM size, list length).

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.

// ❌ 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

// ❌ 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

// ❌ 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

Open React DevTools → Profiler tab.

Click Record, perform the slow interaction, then Stop.

Switch to Flamegraph view; any bar labeled with a component and time > ~16 ms is a candidate.

Use Ranked chart to sort by self render time and target the top offenders.

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.