React Native / Expo Performance

This file extends common/performance.md with React Native / Expo specific content.

Rendering

• Memoize expensive components with React.memo; memoize callbacks/values passed to children with useCallback/useMemo only where they prevent real re-renders.
• Keep component state local and narrow — lifting state too high re-renders large subtrees.
• Avoid creating new objects/arrays/functions inline in props on hot paths; they break memoization.
• Split large screens so a state change re-renders the smallest possible subtree.

Lists

• Use FlatList/SectionList, or FlashList (Shopify) for large or heterogeneous lists.
• Provide keyExtractor, a memoized renderItem, and stable item heights when possible (getItemLayout).
• Tune initialNumToRender, windowSize, maxToRenderPerBatch for heavy rows.
• Never render large data sets with .map() inside a ScrollView.

Images & Assets

• Use expo-image for caching, priority, and placeholders; serve appropriately sized images.
• Avoid loading full-resolution images into small thumbnails.

Animations

• Prefer react-native-reanimated (runs on the UI thread) over the JS-driven Animated API.
• For legacy Animated, set useNativeDriver: true where supported.
• Keep heavy computation off the JS thread; offload to Reanimated worklets or native modules.

Runtime & Build

• Build on the New Architecture (Fabric + TurboModules). It is the default in recent Expo SDKs (opt-out still available on SDK 53–54) and is mandatory — cannot be disabled — from SDK 55+. Verify every native dependency is New-Arch compatible before shipping.
• Ensure Hermes is enabled (default in modern Expo) for faster startup and lower memory.
• Defer non-critical work after first paint; lazy-load heavy screens/modules.
• Use InteractionManager.runAfterInteractions for work that can wait until animations finish.

Measuring

• Profile with the React DevTools profiler, the Hermes sampling profiler, and the in-app performance monitor. (Avoid Flipper — it is deprecated and not supported on the New Architecture.)
• Watch for: long lists without virtualization, oversized images, frequent full-tree re-renders, and synchronous work on the JS thread.