Ecc React Performance

React Performance

Performance optimization patterns for React 18/19 and Next.js, adapted from Vercel Labs react-best-practices (MIT, v1.0.0). This skill organizes rules by priority and provides decision-tree guidance for active code review and refactoring.

When to Activate

• Writing or reviewing React/Next.js code for performance
• Diagnosing slow page loads, slow interactions, or high CPU on the client
• Auditing bundle size or Lighthouse Core Web Vitals regressions
• Removing waterfalls in Server Components / API routes
• Reducing client-side re-renders
• Optimizing long lists, animations, or hydration
• Auditing optimization choices in PRs touching app/, pages/, components/, or data layers

Priority Index

1. Eliminating Waterfalls (CRITICAL)

"Waterfalls are the #1 performance killer" — every sequential await adds full network latency.

Cheap conditions before await

Check sync conditions (props, env, hardcoded flags) before awaiting remote data.

// INCORRECT
async function Page({ id }: { id: string }) {
  const flag = await getFlag("show-page");
  if (!flag || !id) return null;
  const data = await getData(id);
  // ...
}

// CORRECT — short-circuit on cheap sync condition first
async function Page({ id }: { id: string }) {
  if (!id) return null;
  const flag = await getFlag("show-page");
  if (!flag) return null;
  const data = await getData(id);
}

Defer awaits until used

Move await into the branch that uses it.

// INCORRECT — awaits before deciding it needs the data
const user = await getUser(id);
if (mode === "guest") return renderGuest();
return renderUser(user);

// CORRECT
if (mode === "guest") return renderGuest();
const user = await getUser(id);
return renderUser(user);

Promise.all for independent work

// INCORRECT — sequential
const user = await getUser(id);
const posts = await getPosts(id);
const followers = await getFollowers(id);

// CORRECT — parallel
const [user, posts, followers] = await Promise.all([
  getUser(id),
  getPosts(id),
  getFollowers(id),
]);

Partial dependencies — start early, await late

// CORRECT — kick off all promises, await only when each result is needed
const userP = getUser(id);
const postsP = getPosts(id);
const profile = await getProfile(id);
if (profile.private) return null;
const [user, posts] = await Promise.all([userP, postsP]);

Suspense for streaming

Push <Suspense> boundaries close to the data so the page paints what it can while slower sub-trees stream in. The trade-off: layout shift when content arrives — reserve space (skeleton or min-height).

Server Components: parallel through composition

// INCORRECT — sibling awaits run sequentially inside one component
export default async function Page() {
  const user = await getUser();
  const cart = await getCart();
  return <View user={user} cart={cart} />;
}

// CORRECT — split into children, React runs them in parallel
export default async function Page() {
  return (
    <View>
      <UserSection />
      <CartSection />
    </View>
  );
}

2. Bundle Size Optimization (CRITICAL)

Direct imports, not barrels

Barrel index.ts files force the bundler to walk the entire module graph even when tree-shaking removes most of it. Direct imports save 200-800ms of first-load JS in many real-world apps.

// INCORRECT
import { Button, Card, Modal } from "@/components";

// CORRECT
import { Button } from "@/components/Button";
import { Card } from "@/components/Card";
import { Modal } from "@/components/Modal";

Next.js 13.5+ has Optimize Package Imports that automates this for listed packages — use it; manual direct imports still required for non-listed libs.

Statically analyzable paths

// INCORRECT — defeats bundler/trace analysis
const mod = await import(`./pages/${name}`);

// CORRECT — explicit per branch
const mod = name === "home" ? await import("./pages/home") : await import("./pages/about");

Dynamic imports for heavy components

import dynamic from "next/dynamic";

const HeavyChart = dynamic(() => import("./HeavyChart"), {
  loading: () => <Skeleton />,
  ssr: false, // when client-only
});

Defer third-party scripts

Load analytics, logging, support widgets AFTER hydration. Use next/script with strategy="afterInteractive" (default) or "lazyOnload".

Conditional module loading

if (user.role === "admin") {
  const { AdminPanel } = await import("./admin/AdminPanel");
  // ...
}

Preload on hover/focus

Trigger <link rel="preload"> or import() on hover so the bundle is in cache by the time the user clicks.

3. Server-Side Performance (HIGH)

Authenticate Server Actions like API routes

Every "use server" function is a public endpoint. Authenticate AND authorize inside the action — never rely on the calling Client Component's gating.

"use server";
export async function deleteUser(formData: FormData) {
  const session = await getSession();
  if (!session?.user) throw new Error("Unauthorized");
  const targetId = String(formData.get("id"));
  if (session.user.role !== "admin" && session.user.id !== targetId) {
    throw new Error("Forbidden");
  }
  await db.user.delete({ where: { id: targetId } });
}

React.cache() for per-request deduplication

import { cache } from "react";

export const getUser = cache(async (id: string) => {
  return db.user.findUnique({ where: { id } });
});

React.cache dedupes within a single request. Calling getUser("1") from three Server Components in the same render = one DB query.

LRU cache for cross-request data

For data that does NOT change per request (config, lookup tables), cache outside React with an LRU cache or unstable_cache.

Avoid duplicate serialization in RSC props

When a Server Component renders the same data into multiple Client Components, the data is serialized once per consumer. Lift the Client Component up and pass children.

Hoist static I/O to module scope

// CORRECT — runs once at module load
const fontData = readFileSync(fontPath);

export async function Page() {
  return <Banner font={fontData} />;
}

No mutable module-level state in RSC/SSR

Module state on the server is shared across all requests — a race condition between users. Use request-scoped storage (headers(), cookies(), async context) instead.

Minimize data passed to Client Components

Only serialize what the Client needs. Strip fields, paginate, project columns at the DB layer.

Parallelize nested fetches with Promise.all per item

const users = await getUsers();
const enriched = await Promise.all(
  users.map(async (u) => ({ ...u, posts: await getPostsFor(u.id) })),
);

Use after() for non-blocking work

Next.js 15 after() runs work after the response is sent — logging, cache warming, analytics.

import { after } from "next/server";
export async function GET() {
  const data = await getData();
  after(() => logAnalytics(data));
  return Response.json(data);
}

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

SWR / TanStack Query for deduplication

Multiple components calling useUser(id) should share one network request and one cache entry. Use SWR or TanStack Query — never roll your own useEffect + fetch for shared data.

Deduplicate global event listeners

// INCORRECT — every component adds its own
useEffect(() => {
  window.addEventListener("scroll", handler);
  return () => window.removeEventListener("scroll", handler);
}, []);

// CORRECT — single shared listener via a hook + global subject
const useScroll = createScrollHook(); // singleton subject under the hood

Passive listeners for scroll

window.addEventListener("scroll", handler, { passive: true });

Improves scrolling smoothness; the listener cannot preventDefault().

localStorage: version + minimize

• Always store a version field; bump on schema change and migrate or discard old data
• Keep payloads small — localStorage is synchronous and blocks main thread

5. Re-render Optimization (MEDIUM)

Don't subscribe to state used only in callbacks

// INCORRECT — re-renders every time count changes
const count = useStore((s) => s.count);
const handler = () => doSomething(count);

// CORRECT — read once on call
const handler = () => {
  const count = useStore.getState().count;
  doSomething(count);
};

Extract expensive work into memoized components

// CORRECT — child re-renders only when `items` changes
const Heavy = memo(function Heavy({ items }: { items: Item[] }) {
  return <Chart data={transform(items)} />;
});

Hoist default non-primitive props

// INCORRECT — new array each render breaks memo
<List items={items ?? []} />

// CORRECT
const EMPTY: Item[] = [];
<List items={items ?? EMPTY} />

Primitive dependencies in effects

// INCORRECT — new object identity every render
useEffect(() => {}, [{ id, name }]);

// CORRECT — primitives
useEffect(() => {}, [id, name]);

Subscribe to derived booleans, not raw values

// INCORRECT — re-renders for any cart change
const cart = useStore((s) => s.cart);
const hasItems = cart.length > 0;

// CORRECT — re-renders only when emptiness flips
const hasItems = useStore((s) => s.cart.length > 0);

Derive during render, never via useEffect

// INCORRECT
const [full, setFull] = useState("");
useEffect(() => setFull(`${first} ${last}`), [first, last]);

// CORRECT
const full = `${first} ${last}`;

Functional setState for stable callbacks

// CORRECT
const increment = useCallback(() => setCount((c) => c + 1), []);

Lazy state initializer for expensive values

const [tree] = useState(() => parseTree(largeInput));

Avoid memo for simple primitives

useMemo(() => x + 1, [x]) is overhead. Memo earns its keep on object identity and expensive computation.

Split hooks with independent deps

// INCORRECT — both selectors re-run if either source changes
const { a, b } = useSomething(source1, source2);

// CORRECT
const a = useA(source1);
const b = useB(source2);

Move interaction logic into event handlers

Event handlers run only on the user action — useEffect re-runs whenever deps change.

startTransition for non-urgent updates

const [pending, startTransition] = useTransition();
startTransition(() => setFilters(newFilters));

useDeferredValue for expensive renders

const deferredQuery = useDeferredValue(query);
const results = useMemo(() => expensiveSearch(deferredQuery), [deferredQuery]);

useRef for transient frequent values

For values that change often but should not trigger re-render (timestamps, last-key, accumulators).

Don't define components inside components

// INCORRECT — Inner is a new component on every Outer render
function Outer() {
  const Inner = () => <span />;
  return <Inner />;
}

Each render makes a new Inner type, defeating reconciliation and unmounting children.

6. Rendering Performance (MEDIUM)

Animate the wrapper, not the SVG

Transforming a <div> wrapper around an SVG is GPU-accelerated; transforming the SVG itself triggers paint.

content-visibility: auto for long lists

.row { content-visibility: auto; contain-intrinsic-size: auto 80px; }

Browser skips offscreen rendering — major win for lists with hundreds of rows.

Hoist static JSX

const STATIC_HEADER = <h1>Title</h1>;
function Page() {
  return <>{STATIC_HEADER}<Body /></>;
}

SVG: reduce coordinate precision

d="M10.123456,20.654321" → d="M10.12,20.65". Each digit costs bytes; the visual difference is sub-pixel.

Hydration no-flicker via inline script

For values needed before hydration (theme, locale), inline a <script> that sets document.documentElement.dataset.* before React mounts.

Suppress expected hydration mismatches narrowly

<time suppressHydrationWarning>{new Date().toLocaleString()}</time>

Use ONLY for known-divergent leaf nodes — never on a tree containing other children.

<Activity> for show/hide instead of mount/unmount

React 19 <Activity mode="visible|hidden"> keeps tree state and effects mounted but hides — cheaper than unmount/remount for tabs and accordions.

Ternary over && for conditional render

// INCORRECT — `0` renders as text node
{count && <Badge>{count}</Badge>}

// CORRECT
{count > 0 ? <Badge>{count}</Badge> : null}

useTransition for loading states

Pair startTransition with the action; React shows the previous UI as isPending while the next state computes.

React DOM resource hints

import { preload, preconnect } from "react-dom";
preload("/api/critical", { as: "fetch" });
preconnect("https://api.example.com");

defer / async on <script> tags

defer for ordered execution after DOMContentLoaded; async for fire-and-forget.

7. JavaScript Performance (LOW-MEDIUM)

• Batch DOM/CSS changes — apply via class swap or cssText, not property-by-property
• Map for repeated lookups — O(1) vs O(n) linear scan
• Cache property access in loops — const len = arr.length
• Memoize pure functions — module-level Map<key, result>
• Cache localStorage reads — sync API; one read per render
• Combine filter().map() into one pass — flatMap or single for
• Check array length first before expensive comparisons
• Early return from functions
• Hoist RegExp out of loops — compilation is not free
• Loop for min/max instead of sort() — O(n) vs O(n log n)
• Set/Map for membership — O(1) vs Array.includes O(n)
• toSorted() over mutation when immutability matters
• flatMap to map and filter in one pass
• requestIdleCallback for non-critical work

8. Advanced Patterns (LOW)

useEffectEvent deps

Values from useEffectEvent are stable — do NOT add them to effect deps.

Event handler refs

For stable callbacks passed to memoized children:

const handlerRef = useRef(handler);
useEffect(() => { handlerRef.current = handler; });
const stable = useCallback((arg) => handlerRef.current(arg), []);

Init once per app load

For module-level singletons (telemetry, logger), guard with a module-scope flag — not useEffect.

useLatest for stable callback refs

function useLatest<T>(value: T) {
  const ref = useRef(value);
  ref.current = value;
  return ref;
}

Automated Tools

Many of these rules are now automated:

• Next.js 13.5+ Optimize Package Imports — barrel import optimization
• React Compiler (RFC, in canary) — auto-memoization
• Turbopack — faster builds, better tree-shaking
• Bundle Analyzer (@next/bundle-analyzer) — visualize first-load JS

When the project ships React Compiler, demote rerender-* manual memoization rules to "review-only" — the compiler handles them. Manual useMemo/useCallback becomes unnecessary noise.

Lighthouse / Web Vitals Mapping

Related

• Skills: react-patterns, react-testing, frontend-patterns, accessibility, nextjs-turbopack
• Rules: rules/react/
• Agents: react-reviewer enforces these rules in code review; react-build-resolver handles related build failures
• Commands: /react-review, /react-build, /react-test

Attribution

Adapted from Vercel Labs react-best-practices skill (MIT License, copyright Vercel Engineering, v1.0.0 January 2026). Source: https://github.com/vercel-labs/agent-skills/tree/main/skills/react-best-practices.

This skill restructures and adapts the original 70-rule catalog into a single navigable reference. For the full original ruleset with extended examples, see the upstream repository.