Agent Skills: React Best Practices - Performance Optimisation

React Best Practices - Performance Optimisation

Comprehensive performance optimisation guide for React and Next.js applications with 40+ rules organised by impact level. Designed to help developers eliminate performance bottlenecks and follow best practices.

Quick reference

Critical priorities

1. Defer await until needed - Move awaits into branches where they're used
2. Use Promise.all() - Parallelize independent async operations
3. Avoid barrel imports - Import directly from source files
4. Dynamic imports - Lazy-load heavy components
5. Strategic Suspense - Stream content while showing layout

Common patterns

Parallel data fetching:

const [user, posts, comments] = await Promise.all([
  fetchUser(),
  fetchPosts(),
  fetchComments()
])

Direct imports:

// ❌ Loads entire library
import { Check } from 'lucide-react'

// ✅ Loads only what you need
import Check from 'lucide-react/dist/esm/icons/check'

Dynamic components:

import dynamic from 'next/dynamic'

const MonacoEditor = dynamic(
  () => import('./monaco-editor'),
  { ssr: false }
)

Using the guidelines

The guidelines live in the references folder at two depths - pick the one that fits the task:

• references/react-performance-guidelines.md: the complete guide with all rules, code examples, and impact analysis. Read this for a full review or audit.
• references/rules/: one file per rule, named <category>-<rule>.md (e.g. async-parallel.md, bundle-barrel-imports.md). Read individual files when working on a specific problem - the category pointers below map each rule to its file.

Each rule includes:

• Incorrect/correct code comparisons
• Specific impact metrics
• When to apply the optimisation
• Real-world examples

Categories overview

1. Eliminating Waterfalls (CRITICAL)

Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Rules: references/rules/async-*.md

• Defer await until needed - async-defer-await.md
• Dependency-based parallelization - async-dependencies.md
• Prevent waterfall chains in API routes - async-api-routes.md
• Promise.all() for independent operations - async-parallel.md
• Strategic Suspense boundaries - async-suspense-boundaries.md

2. Bundle Size Optimisation (CRITICAL)

Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint. Rules: references/rules/bundle-*.md

• Avoid barrel file imports - bundle-barrel-imports.md
• Conditional module loading - bundle-conditional.md
• Defer non-critical third-party libraries - bundle-defer-third-party.md
• Dynamic imports for heavy components - bundle-dynamic-imports.md
• Preload based on user intent - bundle-preload.md

3. Server-Side Performance (HIGH)

Optimise server-side rendering and data fetching. Rules: references/rules/server-*.md

• Cross-request LRU caching - server-cache-lru.md
• Minimize serialization at RSC boundaries - server-serialization.md
• Parallel data fetching with component composition - server-parallel-fetching.md
• Per-request deduplication with React.cache() - server-cache-react.md

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

Automatic deduplication and efficient data fetching patterns. Rules: references/rules/client-*.md

• Deduplicate global event listeners - client-event-listeners.md
• Use SWR for automatic deduplication - client-swr-dedup.md

5. Re-render Optimisation (MEDIUM)

Reduce unnecessary re-renders to minimize wasted computation. Rules: references/rules/rerender-*.md

• Defer state reads to usage point - rerender-defer-reads.md
• Extract to memoized components - rerender-memo.md
• Narrow effect dependencies - rerender-dependencies.md
• Subscribe to derived state - rerender-derived-state.md
• Use lazy state initialization - rerender-lazy-state-init.md
• Use transitions for non-urgent updates - rerender-transitions.md

6. Rendering Performance (MEDIUM)

Optimise the browser rendering process. Rules: references/rules/rendering-*.md

• Animate SVG wrapper instead of SVG element - rendering-animate-svg-wrapper.md
• CSS content-visibility for long lists - rendering-content-visibility.md
• Hoist static JSX elements - rendering-hoist-jsx.md
• Optimise SVG precision - rendering-svg-precision.md
• Prevent hydration mismatch without flickering - rendering-hydration-no-flicker.md
• Use Activity component for show/hide - rendering-activity.md
• Use explicit conditional rendering - rendering-conditional-render.md

7. JavaScript Performance (LOW-MEDIUM)

Micro-optimisations for hot paths. Rules: references/rules/js-*.md

• Batch DOM CSS changes - js-batch-dom-css.md
• Build index maps for repeated lookups - js-index-maps.md
• Cache property access in loops - js-cache-property-access.md
• Cache repeated function calls - js-cache-function-results.md
• Cache storage API calls - js-cache-storage.md
• Combine multiple array iterations - js-combine-iterations.md
• Early length check for array comparisons - js-length-check-first.md
• Early return from functions - js-early-exit.md
• Hoist RegExp creation - js-hoist-regexp.md
• Use loop for min/max instead of sort - js-min-max-loop.md
• Use Set/Map for O(1) lookups - js-set-map-lookups.md
• Use toSorted() instead of sort() - js-tosorted-immutable.md

8. Advanced Patterns (LOW)

Specialized techniques for edge cases. Rules: references/rules/advanced-*.md

• Store event handlers in refs - advanced-event-handler-refs.md
• useLatest for stable callback refs - advanced-use-latest.md

Implementation approach

When optimising a React application:

1. Profile first: Use React DevTools Profiler and browser performance tools to identify bottlenecks
2. Focus on critical paths: Start with eliminating waterfalls and reducing bundle size
3. Measure impact: Verify improvements with metrics (LCP, TTI, FID)
4. Apply incrementally: Don't over-optimise prematurely
5. Test thoroughly: Ensure optimisations don't break functionality

Key metrics to track

• Time to Interactive (TTI): When page becomes fully interactive
• Largest Contentful Paint (LCP): When main content is visible
• First Input Delay (FID): Responsiveness to user interactions
• Cumulative Layout Shift (CLS): Visual stability
• Bundle size: Initial JavaScript payload
• Server response time: TTFB for server-rendered content

Common pitfalls to avoid

❌ Don't:

• Use barrel imports from large libraries
• Block parallel operations with sequential awaits
• Re-render entire trees when only part needs updating
• Load analytics/tracking in the critical path
• Mutate arrays with .sort() instead of .toSorted()
• Create RegExp or heavy objects inside render

✅ Do:

• Import directly from source files
• Use Promise.all() for independent operations
• Memoize expensive components
• Lazy-load non-critical code
• Use immutable array methods
• Hoist static objects outside components

Resources

• React Documentation
• Next.js Documentation
• SWR Documentation
• Vercel Bundle Optimisation
• Vercel Dashboard Performance
• better-all Library
• node-lru-cache

Version history

v0.1.0 (January 2026)

• Initial release from Vercel Engineering
• 40+ performance rules across 8 categories
• Comprehensive code examples and impact analysis