Vercel React Best Practices

Measurement-led React and Next.js performance skill for routing slow-page work into the right packet before refactoring. The canonical focus is waterfalls, bundle weight, RSC or client-boundary mistakes, hydration/script cost, and rerender churn; the deep AGENTS.md remains available, but day-to-day use should start from the lighter reference bundle and route-outs.

When to use this skill

Reference these guidelines when:

Writing new React components or Next.js pages
Implementing data fetching (client or server-side)
Reviewing code for performance issues
Refactoring existing React/Next.js code
Optimizing bundle size or load times

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Eliminating Waterfalls	CRITICAL	`async-`
2	Bundle Size Optimization	CRITICAL	`bundle-`
3	Server-Side Performance	HIGH	`server-`
4	Client-Side Data Fetching	MEDIUM-HIGH	`client-`
5	Re-render Optimization	MEDIUM	`rerender-`
6	Rendering Performance	MEDIUM	`rendering-`
7	JavaScript Performance	LOW-MEDIUM	`js-`
8	Advanced Patterns	LOW	`advanced-`

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-api-routes - Start promises early, await late in API routes
async-suspense-boundaries - Use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

bundle-barrel-imports - Import directly, avoid barrel files
bundle-dynamic-imports - Use next/dynamic 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. Server-Side Performance (HIGH)

server-cache-react - Use React.cache() for per-request deduplication
server-cache-lru - Use LRU cache for cross-request caching
server-serialization - Minimize data passed to client components
server-parallel-fetching - Restructure components to parallelize fetches
server-after-nonblocking - Use after() for non-blocking operations

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

client-swr-dedup - Use SWR for automatic request deduplication
client-event-listeners - Deduplicate global event listeners

5. 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-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

6. Rendering Performance (MEDIUM)

rendering-animate-svg-wrapper - Animate div wrapper, not SVG element
rendering-content-visibility - Use content-visibility for long lists
rendering-hoist-jsx - Extract static JSX outside components
rendering-svg-precision - Reduce SVG coordinate precision
rendering-hydration-no-flicker - Use inline script for client-only data
rendering-activity - Use Activity component for show/hide
rendering-conditional-render - Use ternary, not && for conditionals

7. JavaScript Performance (LOW-MEDIUM)

js-batch-dom-css - Group CSS changes via classes or cssText
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

8. Advanced Patterns (LOW)

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

Instructions

Step 1: Classify the primary bottleneck before suggesting fixes

Start with references/perf-triage-modes.md and put the task into one dominant packet:

waterfalls / async sequencing
bundle weight / lazy loading
RSC / server-client boundary mistakes
rerender churn / memoization / context spread
hydration or third-party script cost
measurement-first unknowns

Step 2: Use the smallest support packet that answers the question

references/perf-triage-modes.md — choose the main React / Next.js perf mode and first fixes
references/measurement-and-tooling-checklist.md — decide what to measure with React Profiler, bundle analyzers, Web Vitals, or PR budget tooling
references/boundaries-and-route-outs.md — keep the boundary with state-management, web-accessibility, responsive-design, and performance-optimization explicit
AGENTS.md — deep rule catalog when a code generation or large refactor pass needs the full 45-rule reference

Step 3: Prefer measurement-led fixes over blanket cargo culting

Use the measurement checklist before recommending broad memoization or cache-heavy rewrites. React Compiler, bundle analyzers, and web vitals instrumentation have changed which old React performance heuristics are still worth applying blindly.

Step 4: Prioritize high-impact fixes first

Remove waterfalls and late-started async work.
Cut route JS and heavy client boundaries.
Fix hydration or script-placement mistakes.
Address rerender churn once the larger loading and boundary costs are understood.

Step 5: Route adjacent work out instead of absorbing everything

App-wide state model choice → ../state-management/SKILL.md
Accessibility-specific failures → ../web-accessibility/SKILL.md
Layout adaptation / overflow / reflow work → ../responsive-design/SKILL.md
Non-React or cross-stack bottlenecks → ../performance-optimization/SKILL.md

Examples

Promise.all for Independent Operations (CRITICAL)

// ❌ Sequential: 3 round trips
const user = await fetchUser()
const posts = await fetchPosts()
const comments = await fetchComments()

// ✅ Parallel: 1 round trip
const [user, posts, comments] = await Promise.all([
  fetchUser(),
  fetchPosts(),
  fetchComments()
])

Avoid Barrel File Imports (CRITICAL)

// ❌ Imports entire library (200-800ms import cost)
import { Check, X, Menu } from 'lucide-react'

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

Dynamic Imports for Heavy Components (CRITICAL)

// ❌ Monaco bundles with main chunk ~300KB
import { MonacoEditor } from './monaco-editor'

// ✅ Monaco loads on demand
import dynamic from 'next/dynamic'
const MonacoEditor = dynamic(
  () => import('./monaco-editor').then(m => m.MonacoEditor),
  { ssr: false }
)

Use Functional setState (MEDIUM)

// ❌ Requires state as dependency, stale closure risk
const addItems = useCallback((newItems) => {
  setItems([...items, ...newItems])
}, [items])

// ✅ Stable callback, no stale closures
const addItems = useCallback((newItems) => {
  setItems(curr => [...curr, ...newItems])
}, [])

Best practices

Start with the lightest support packet that matches the problem; use AGENTS.md only when the smaller references are not enough.
Prefer evidence-led fixes: React Profiler, bundle analyzers, and Web Vitals should shape the recommendation before you prescribe memoization or caching.
Treat React Compiler as a live constraint on old memoization folklore; do not default to “wrap everything in memo”.
Keep React-specific advice separate from broader frontend concerns: use state-management for app-wide state choices, web-accessibility for WCAG remediation, and performance-optimization for non-React-wide bottlenecks.
Escalate to the compatibility alias vercel-react-best-practices only when an existing workflow explicitly requires that exact name.
Keep PR-budget and bundle-diff workflows in mind for real team usage; the skill should help with operational audits, not just code-level micro-fixes.

Constraints

Required Rules (MUST)

Eliminate waterfalls: Use Promise.all, Suspense
Bundle optimization: Prohibit barrel imports, use dynamic imports
RSC boundaries: Serialize only the data you need

Prohibited (MUST NOT)

Sequential await: Do not run independent fetches sequentially
Array mutations: Use toSorted() instead of sort()
Inline objects in React.cache: Causes cache misses

References

references/perf-triage-modes.md
references/measurement-and-tooling-checklist.md
references/boundaries-and-route-outs.md
AGENTS.md
React Documentation
Next.js Documentation
SWR
better-all
Vercel Blog: Optimizing Package Imports
Vercel Blog: Dashboard Performance

react-best-practices