performance

Performance Skill

Applicability

• Platforms: iOS and Android
• React Native: 0.76+ (New Architecture interop assumed unless a checklist item says otherwise)

When to Use

• A screen drops frames, stutters, or feels slow
• Reviewing UI-heavy code or long lists before merge
• Profiling re-renders or memory growth over a session
• Adding animations or large images

Guidance

Rendering

• No inline functions passed as props to memoised children (breaks React.memo)
• Heavy computations wrapped in useMemo
• Callbacks passed as props wrapped in useCallback
• Inline styles extracted to StyleSheet.create (no new object per render)
• No unnecessary re-renders (verify with a tool such as why-did-you-render)

Incorrect:

// New function every render, breaks React.memo on the child
<UserCard onPress={() => handlePress(user.id)} />

Correct:

const handleUserPress = useCallback((id: string) => {
  handlePress(id);
}, [handlePress]);

<UserCard onPress={handleUserPress} userId={user.id} />

Lists

• Long lists use a virtualised, high-performance list (e.g. FlashList), not a plain FlatList
• Large data sets paginate or use infinite scroll, never render the full array
• List items are memoised where appropriate

Images

• Images lazy loaded below the fold
• Images use a modern compressed format (e.g. WebP) at correct dimensions (no oversized assets)

Animations

• Complex animations run on the UI thread (e.g. Reanimated), not the JS thread
• No animation work blocking interaction

Loading & Imports

• Screens lazy loaded per navigation stack
• No full-library imports for a single function (import only what is used)
• No synchronous storage reads on mount that block the first render

Memory & Resources

• Async operations cancelled on unmount
• Timers and intervals cleared on unmount
• Event listeners removed on cleanup
• Caches have a max size and an eviction policy

Incorrect:

useEffect(() => {
  const interval = setInterval(pollStatus, 5000);
  // No cleanup — keeps running after unmount
}, []);

Correct:

useEffect(() => {
  const interval = setInterval(pollStatus, 5000);
  return () => clearInterval(interval);
}, []);

Anti-Patterns

Anti-Pattern	Risk	Fix
FlatList for long lists	Frame drops	Use FlashList or equivalent
Frequent reads from slow storage	Blocks JS thread	Use a fast key-value store (e.g. MMKV)
Animated API for complex animations	JS-thread jank	Use Reanimated
Inline styles in render	New object each render, breaks memo	StyleSheet.create
Over-memoising trivial components	Adds overhead with no benefit	Measure first, memoise hotspots

Pitfalls

• Over-memoising simple components costs more than it saves — measure before optimising.
• useMemo or useCallback with the wrong dependency array silently returns stale values.
• Memory leaks compound over a long session (users keep apps open for hours), so cleanup matters even for short-lived screens.
• An AbortController must be created per request, not shared across calls.