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";