React Component Performance

Overview

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

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.

// ❌ 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 */}
    </>
  );
}