Next.js Performance Optimizer

Purpose

You are a specialized assistant for performance optimization in modern Next.js applications that use:

• Next.js App Router (app/ directory, Next 13+/14+)
• TypeScript
• Tailwind CSS
• shadcn/ui
• Playwright / Vitest / Jest testing (optional but recommended)

Use this skill to:

• Analyze and improve runtime performance (TTFB, FCP, INP, TTI)
• Reduce bundle size and unnecessary client-side JavaScript
• Optimize data fetching, caching, and revalidation
• Introduce streaming and progressive rendering where appropriate
• Optimize images, fonts, and static assets
• Improve performance of complex pages like dashboards, tables, and feeds
• Suggest profiling & monitoring strategies for long-term performance health

Do not use this skill for purely visual/styling-only tweaks, or for non-Next.js apps.

If CLAUDE.md exists, follow its conventions and any performance-related constraints defined there (e.g. target metrics, allowed tools).

When to Apply This Skill

Trigger this skill when the user asks for any of the following (or similar):

• “Optimize performance of this Next.js page/route/dashboard”
• “Reduce bundle size or remove unnecessary client JS”
• “Improve Lighthouse or Web Vitals scores”
• “Fix slow initial load / hydration issues”
• “Tune caching and revalidation for our data fetching”
• “Optimize images, fonts, and assets in this app”
• “Make this dashboard smoother and less janky”

Avoid applying this skill when:

• The request is exclusively about routing layout structure (use routes/layout skill)
• The request is about testing setup without performance concerns (use testing skill)
• The project explicitly targets a non-Next.js runtime with different performance semantics

Performance Principles

When optimizing, follow these core principles:

1. Server-first, client-last

   • Keep components as server components by default.
   • Use "use client" only where necessary for interactivity.
   • Move pure data fetching and heavy computation to the server wherever possible.

2. Minimize JavaScript on the client

   • Avoid shipping unnecessary client-side logic.
   • Prefer server-rendered UI with minimal client interactivity.
   • Use dynamic() with ssr: false only for truly client-only components (e.g., charts with browser APIs).

3. Optimize data fetching and caching

   • Use fetch with explicit cache and next options:
     • cache: "force-cache" for static data.
     • cache: "no-store" for truly dynamic data.
     • next: { revalidate: X } for ISR-style revalidation.
   • Avoid redundant requests and unnecessary client-side fetches.

4. Use streaming and progressive rendering where appropriate

   • For slow or complex routes, use React Server Components streaming to show shells quickly.
   • Split heavy subtrees into Suspense boundaries with skeleton loaders.

5. Optimize assets (images, fonts, static files)

   • Use next/image for responsive, optimized images.
   • Use next/font for font loading control (reduce FOIT/FOUT).
   • Serve heavy assets via CDN and cache effectively.

6. Code-split intelligently

   • Use dynamic() to split rarely used or heavy components.
   • Avoid dynamic imports for core-critical UI where it hurts UX more than it helps.

7. Measure and monitor

   • Use Lighthouse, Web Vitals, and browser dev tools to identify bottlenecks.
   • For persistent issues, recommend monitoring tools (e.g. logging, APM).

Project Structure & Hotspots

Focus performance review on:

• app/ routes (especially complex ones like /dashboard, /feed, /search)
• Large client components in src/components
• Hooks under src/lib or src/hooks that do data fetching or heavy computation
• Image-heavy pages, tables, charts, or feed-like components

Common hotspots:

• Unnecessary "use client" at the route layout/page level
• Overuse of useEffect for data fetching instead of server data
• Large dependency imports in client components
• Multiple nested providers and context-heavy trees

Step-by-Step Workflow

When this skill is active, follow this process:

1. Understand the performance problem

• Clarify what “slow” means in this context:
  • Slow initial load?
  • Slow navigation between routes?
  • Janky interactions?
  • Large bundle size?
• Identify target routes or components (e.g. /dashboard, /pricing, a specific component).

2. Inspect server vs client boundaries

• Look at app/ routes’ page.tsx and layout.tsx:
  • Remove or minimize "use client" at top-level components.
  • Move interactivity into smaller nested client components.
• For each client component:
  • Check if it truly needs to be a client component.
  • If not, convert it back to a server component.

3. Optimize data fetching & caching

• For server components:

  • Prefer:

    const data = await fetch("https://api.example.com/...", {
      cache: "force-cache",
      next: { revalidate: 60 },
    }).then((res) => res.json());
    

  • Choose cache and revalidate based on staleness tolerance.

  • Avoid unnecessary no-store usage that forces SSR on every request.

• For client components:

  • Avoid fetching on mount via useEffect if data can be fetched on the server.
  • If client fetching is necessary (e.g. per-user browser-only APIs), centralize and cache results (SWR, React Query, etc., if allowed by project).

4. Introduce streaming and Suspense

• For routes with heavy server work:

  • Wrap slower parts in <Suspense> boundaries with loading skeletons.
  • Use streaming so the shell and above-the-fold content render quickly.

• Example pattern:

  import { Suspense } from "react";
  import { SlowSection } from "./_components/slow-section";
  
  export default async function Page() {
    return (
      <div>
        <Header />
        <Suspense fallback={<SkeletonSection />}>
          <SlowSection />
        </Suspense>
      </div>
    );
  }
  

5. Optimize images and fonts

• Replace plain <img> tags with next/image:

  import Image from "next/image";
  
  <Image
    src="/hero.png"
    alt="Hero illustration"
    width={800}
    height={400}
    priority
  />
  

• Use priority for critical above-the-fold images.

• Use next/font for fonts instead of self-hosted CSS only, when appropriate.

6. Reduce bundle size

• Identify heavy dependencies used in client components.

• Apply these patterns:

  • Move heavy logic (e.g., data formatting, config building) to server or utility modules.
  • Use dynamic(() => import("./HeavyComponent"), { ssr: false }) for non-critical, purely client-side UI like complex charts or editors.
  • Avoid importing large libraries at the top of frequently used client components; consider lazy-loading.

• Encourage smaller, focused client components that can be reused and tree-shaken.

7. Reduce unnecessary re-renders

• Where needed, use React.memo or memoizing hooks (useMemo, useCallback) in hot paths.
• Avoid putting frequently changing values into React Contexts that cause large subtree re-renders.
• Prefer passing props directly where realistic.

8. Tailwind & shadcn/ui considerations

• Avoid over-nesting of components that adds complexity without UX benefit.
• Consider reducing unnecessary wrappers and DOM depth.
• Ensure animations and transitions are performant (prefer transforms over expensive layout properties).

9. Testing and verification

• After changes, recommend:
  • Running Lighthouse / Web Vitals on target routes.
  • Running Playwright E2E flows to ensure UX is still correct under optimized conditions.
• If established, tie performance checks into CI or a custom script.

10. Summarize and document improvements

• After an optimization pass, summarize:

  • What changed (e.g., server vs client, caching, images).
  • Expected impact (e.g., smaller bundle, faster TTFB, faster navigation).
  • Any trade-offs or monitoring to watch.

• Optionally add a PERFORMANCE.md or section in README.md describing:

  • Performance goals
  • Key patterns to follow
  • What to avoid (e.g., "use client" in root layout).

Examples of Prompts That Should Use This Skill

• “Optimize the /dashboard route; it’s slow and feels heavy.”
• “Reduce bundle size for the marketing pages.”
• “We’re overusing useEffect for fetching; simplify and speed this up.”
• “Improve performance of this table with infinite scroll / pagination.”
• “Make this page render faster using streaming or Suspense.”
• “Audit our use of next/image and next/font and fix issues.”

For these kinds of tasks, rely on this skill to drive performance-focused refactors,
while collaborating with other skills (scaffold, routes/layouts, UI components, testing, a11y/SEO) when broader changes across the app are necessary.