React 19+ Performance Best Practices

Comprehensive performance optimization guide for React 19+ applications. Contains 35+ rules across 7 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• •Writing new React components or hooks
• •Implementing data fetching (client-side)
• •Reviewing code for performance issues
• •Refactoring existing React code
• •Optimizing bundle size or load times
• •Working with React 19+ features (use(), Activity, etc.)

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• •async-defer-await - Move await into branches where actually used
• •async-parallel - Use Promise.all() for independent operations
• •async-dependencies - Use better-all for partial dependencies
• •async-suspense-boundaries - Use Suspense + use() hook to stream content

2. Bundle Size Optimization (CRITICAL)

• •bundle-barrel-imports - Import directly, avoid barrel files
• •bundle-dynamic-imports - Use React.lazy() for heavy components
• •bundle-defer-third-party - Load analytics/logging after hydration
• •bundle-conditional - Load modules only when feature is activated
• •bundle-preload - Preload on hover/focus for perceived speed

3. Client-Side Data Fetching (MEDIUM-HIGH)

• •client-swr-dedup - Use SWR/TanStack Query for automatic request deduplication
• •client-event-listeners - Deduplicate global event listeners
• •client-passive-event-listeners - Use passive listeners for scroll performance
• •client-localstorage-schema - Version and minimize localStorage data

4. Re-render Optimization (MEDIUM)

• •rerender-defer-reads - Don't subscribe to state only used in callbacks
• •rerender-memo - Extract expensive work into memoized components
• •rerender-memo-with-default-value - Extract default non-primitive values to constants
• •rerender-simple-expression-in-memo - Don't wrap simple expressions in useMemo
• •rerender-dependencies - Use primitive dependencies in effects
• •rerender-derived-state - Subscribe to derived booleans, not raw values
• •rerender-functional-setstate - Use functional setState for stable callbacks
• •rerender-lazy-state-init - Pass function to useState for expensive values
• •rerender-transitions - Use startTransition for non-urgent updates

5. Rendering Performance (MEDIUM)

• •rendering-activity - Use Activity component for show/hide (React 19)
• •rendering-animate-svg-wrapper - Animate div wrapper, not SVG element
• •rendering-conditional-render - Use ternary, not && for conditionals
• •rendering-content-visibility - Use content-visibility for long lists
• •rendering-hoist-jsx - Extract static JSX outside components
• •rendering-hydration-no-flicker - Prevent hydration mismatch without flickering
• •rendering-svg-precision - Reduce SVG coordinate precision
• •rendering-usetransition-loading - Use useTransition over manual loading states

6. JavaScript Performance (LOW-MEDIUM)

• •js-batch-dom-css - Avoid layout thrashing with batched DOM operations
• •js-index-maps - Build Map for repeated lookups
• •js-cache-property-access - Cache object properties in loops
• •js-cache-function-results - Cache function results in module-level Map
• •js-cache-storage - Cache localStorage/sessionStorage reads
• •js-combine-iterations - Combine multiple filter/map into one loop
• •js-length-check-first - Check array length before expensive comparison
• •js-early-exit - Return early from functions
• •js-hoist-regexp - Hoist RegExp creation outside loops
• •js-min-max-loop - Use loop for min/max instead of sort
• •js-set-map-lookups - Use Set/Map for O(1) lookups
• •js-tosorted-immutable - Use toSorted() for immutability

7. Advanced Patterns (LOW)

• •advanced-event-handler-refs - Store event handlers in refs
• •advanced-use-latest - useEffectEvent for stable callback refs (experimental)

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-barrel-imports.md
rules/_sections.md

Each rule file contains:

• •Brief explanation of why it matters
• •Incorrect code example with explanation
• •Correct code example with explanation
• •Additional context and references

Full Compiled Document

For the complete guide with all rules expanded: SKILL.md

Key Differences from Next.js Version

This skill focuses on pure React 19+ without framework-specific features:

• •Uses React.lazy() instead of next/dynamic
• •Client-side data fetching with SWR/TanStack Query
• •No server-side rendering optimizations
• •No API route patterns
• •Emphasizes modern React 19 features (use() hook, Activity component)
• •Framework-agnostic bundler configuration (Vite, webpack, etc.)

React 19+ Specific Features

This guide includes React 19-specific optimizations:

• •use() hook - For unwrapping promises in Suspense boundaries
• •Activity component - For preserving state/DOM on visibility toggle
• •Async transitions - Enhanced useTransition with async support
• •Experimental useEffectEvent - For stable callback references