react-native-optimizations

name	react-native-optimizations
description	Provides React Native performance optimization guidelines for FPS, TTI, bundle size, memory leaks, re-renders, and animations. Applies to tasks involving Hermes optimization, JS thread blocking, bridge overhead, FlashList, native modules, or debugging jank and frame drops.
license	MIT
version	1.0.0
author	[Filippo De Silva](https://github.com/FilippoDeSilva)
tags	["react-native","expo","performance","optimization","profiling"]

React Native Best Practices

Overview

Performance optimization guide for React Native applications, covering JavaScript/React, Native (iOS/Android), and bundling optimizations. Based on Callstack's "Ultimate Guide to React Native Optimization".

Skill Format

Each reference file follows a hybrid format for fast lookup and deep understanding:

• Quick Pattern: Incorrect/Correct code snippets for immediate pattern matching
• Quick Command: Shell commands for process/measurement skills
• Quick Config: Configuration snippets for setup-focused skills
• Quick Reference: Summary tables for conceptual skills
• Deep Dive: Full context with When to Use, Prerequisites, Step-by-Step, Common Pitfalls

Impact ratings: CRITICAL (fix immediately), HIGH (significant improvement), MEDIUM (worthwhile optimization)

When to Apply

Reference these guidelines when:

• Debugging slow/janky UI or animations
• Investigating memory leaks (JS or native)
• Optimizing app startup time (TTI)
• Reducing bundle or app size
• Writing native modules (Turbo Modules)
• Profiling React Native performance
• Reviewing React Native code for performance

Security Notes

• Treat shell commands in these references as local developer operations. Review them before running, prefer version-pinned tooling, and avoid piping remote scripts directly to a shell.
• Treat third-party libraries and plugins as dependencies that still require normal supply-chain controls: pin versions, verify provenance, and update through your standard review process.
• Treat Re.Pack code splitting as first-party artifact delivery only. Remote chunks must come from trusted HTTPS origins you control and be pinned to the current app release.

Priority-Ordered Guidelines

Priority	Category	Impact	Prefix
1	FPS & Re-renders	CRITICAL	js-*
2	Bundle Size	CRITICAL	bundle-*
3	TTI Optimization	HIGH	native-*, bundle-*
4	Native Performance	HIGH	native-*
5	Memory Management	MEDIUM-HIGH	js-*, native-*
6	Animations	MEDIUM	js-*

Quick Reference

Optimization Workflow

Follow this cycle for any performance issue: Measure → Optimize → Re-measure → Validate

1. Measure: Capture baseline metrics before changes. For runtime issues, prefer commit timeline, re-render counts, slow components, heaviest-commit breakdown, and startup/TTI when available. Component tree depth or count are optional context, not substitutes.
2. Optimize: Apply the targeted fix from the relevant reference
3. Re-measure: Run the same measurement to get updated metrics
4. Validate: Confirm improvement (e.g., FPS 45→60, TTI 3.2s→1.8s, bundle 2.1MB→1.6MB)

If metrics did not improve, revert and try the next suggested fix.

Review Guardrails

• Check library versions before suggesting API-specific fixes. Example: FlashList v2 deprecates estimatedItemSize, so do not flag it as missing there.
• Do not suggest useMemo or useCallback dependency changes unless behavior is demonstrably incorrect or profiling shows wasted work tied to that value.
• Do not report stale closures speculatively. Show the stale read path, a repro, or profiler evidence before calling it out.
• When profiling a flow, measure the target interaction itself. Do not treat component tree depth or component count as the main performance evidence.

Critical: FPS & Re-renders

Profile first:

# Open React Native DevTools
# Press 'j' in Metro, or shake device → "Open DevTools"

Common fixes:

• Replace ScrollView with FlatList/FlashList for lists
• Use React Compiler for automatic memoization
• Use atomic state (Jotai/Zustand) to reduce re-renders
• Use useDeferredValue for expensive computations

Critical: Bundle Size

Analyze bundle:

npx react-native bundle \
  --entry-file index.js \
  --bundle-output output.js \
  --platform ios \
  --sourcemap-output output.js.map \
  --dev false --minify true

npx source-map-explorer output.js --no-border-checks

Verify improvement after optimization:

# Record baseline size before changes
ls -lh output.js  # e.g., Before: 2.1 MB

# After applying fixes, re-bundle and compare
npx react-native bundle --entry-file index.js --bundle-output output.js \
  --platform ios --dev false --minify true
ls -lh output.js  # e.g., After: 1.6 MB  (24% reduction)

Common fixes:

• Avoid barrel imports (import directly from source)
• Remove unnecessary Intl polyfills only after checking Hermes API and method coverage
• Enable tree shaking (Expo SDK 52+ or Re.Pack)
• Enable R8 for Android native code shrinking

High: TTI Optimization

Measure TTI:

• Use react-native-performance for markers
• Only measure cold starts (exclude warm/hot/prewarm)

Common fixes:

• Disable JS bundle compression on Android (enables Hermes mmap)
• Use native navigation (react-native-screens)
• Preload commonly-used expensive screens before navigating to them

High: Native Performance

Profile native:

• iOS: Xcode Instruments → Time Profiler
• Android: Android Studio → CPU Profiler

Common fixes:

• Use background threads for heavy native work
• Prefer async over sync Turbo Module methods
• Use C++ for cross-platform performance-critical code

References

Full documentation with code examples in references/:

JavaScript/React (js-*)

File	Impact	Description
js-lists-flatlist-flashlist.md	CRITICAL	Replace ScrollView with virtualized lists
js-profile-react.md	MEDIUM	React DevTools profiling
js-measure-fps.md	HIGH	FPS monitoring and measurement
js-memory-leaks.md	MEDIUM	JS memory leak hunting
js-atomic-state.md	HIGH	Jotai/Zustand patterns
js-concurrent-react.md	HIGH	useDeferredValue, useTransition
js-react-compiler.md	HIGH	Automatic memoization
js-animations-reanimated.md	MEDIUM	Reanimated worklets
js-bottomsheet.md	HIGH	Bottom sheet optimization
js-uncontrolled-components.md	HIGH	TextInput optimization

Native (native-*)

File	Impact	Description
native-turbo-modules.md	HIGH	Building fast native modules
native-sdks-over-polyfills.md	HIGH	Native vs JS libraries
native-measure-tti.md	HIGH	TTI measurement setup
native-threading-model.md	HIGH	Turbo Module threads
native-profiling.md	MEDIUM	Xcode/Android Studio profiling
native-platform-setup.md	MEDIUM	iOS/Android tooling guide
native-view-flattening.md	MEDIUM	View hierarchy debugging
native-memory-patterns.md	MEDIUM	C++/Swift/Kotlin memory
native-memory-leaks.md	MEDIUM	Native memory leak hunting
native-android-16kb-alignment.md	CRITICAL	Third-party library alignment for Google Play

Bundling (bundle-*)

File	Impact	Description
bundle-barrel-exports.md	CRITICAL	Avoid barrel imports
bundle-analyze-js.md	CRITICAL	JS bundle visualization
bundle-tree-shaking.md	HIGH	Dead code elimination
bundle-analyze-app.md	HIGH	App size analysis
bundle-r8-android.md	HIGH	Android code shrinking
bundle-hermes-mmap.md	HIGH	Disable bundle compression
bundle-native-assets.md	HIGH	Asset catalog setup
bundle-library-size.md	MEDIUM	Evaluate dependencies
bundle-code-splitting.md	MEDIUM	Re.Pack code splitting

Searching References

# Find patterns by keyword
grep -l "reanimated" references/
grep -l "flatlist" references/
grep -l "memory" references/
grep -l "profil" references/
grep -l "tti" references/
grep -l "bundle" references/

Problem → Skill Mapping

Problem	Start With
App feels slow/janky	js-measure-fps.md → js-profile-react.md
Too many re-renders	js-profile-react.md → js-react-compiler.md
Slow startup (TTI)	native-measure-tti.md → bundle-analyze-js.md
Large app size	bundle-analyze-app.md → bundle-r8-android.md
Memory growing	js-memory-leaks.md or native-memory-leaks.md
Animation drops frames	js-animations-reanimated.md
Bottom sheet jank/re-renders	js-bottomsheet.md → js-animations-reanimated.md
List scroll jank	js-lists-flatlist-flashlist.md
TextInput lag	js-uncontrolled-components.md
Native module slow	native-turbo-modules.md → native-threading-model.md
Native library alignment issue	native-android-16kb-alignment.md

Attribution

Based on "The Ultimate Guide to React Native Optimization" by Callstack.