Performance profiler

Performance Profiler Assistant

When to use this skill

• User asks to run a performance audit
• User mentions Lighthouse or Web Vitals
• User wants to improve page load speed
• User asks about LCP, FID, CLS, or INP
• User wants to profile CPU or memory usage
• User asks to optimize rendering performance

Workflow

• Identify target URL or build to audit
• Run Lighthouse audit
• Collect Core Web Vitals metrics
• Analyze bundle size
• Profile runtime performance if needed
• Generate optimization recommendations
• Prioritize fixes by impact

Instructions

Step 1: Identify Audit Target

For deployed site:

# URL to audit
TARGET_URL="https://example.com"

For local development:

# Ensure dev server is running
npm run dev
TARGET_URL="http://localhost:3000"

For static build analysis:

npm run build
npx serve dist  # or out, build folder

Step 2: Run Lighthouse Audit

CLI audit:

npx lighthouse $TARGET_URL --output=json --output=html --output-path=./lighthouse-report

With specific categories:

npx lighthouse $TARGET_URL --only-categories=performance,accessibility,best-practices,seo --output=json

Mobile emulation (default):

npx lighthouse $TARGET_URL --preset=desktop  # For desktop metrics

Programmatic in Node.js:

import lighthouse from "lighthouse";
import * as chromeLauncher from "chrome-launcher";

const chrome = await chromeLauncher.launch({ chromeFlags: ["--headless"] });
const result = await lighthouse(url, { port: chrome.port });
console.log(result.lhr.categories.performance.score * 100);
await chrome.kill();

Step 3: Interpret Core Web Vitals

Metric	Good	Needs Improvement	Poor
LCP (Largest Contentful Paint)	≤2.5s	2.5s–4s	>4s
INP (Interaction to Next Paint)	≤200ms	200ms–500ms	>500ms
CLS (Cumulative Layout Shift)	≤0.1	0.1–0.25	>0.25
FCP (First Contentful Paint)	≤1.8s	1.8s–3s	>3s
TTFB (Time to First Byte)	≤800ms	800ms–1.8s	>1.8s

Step 4: Analyze Bundle Size

Next.js:

ANALYZE=true npm run build
# Or add to next.config.js:
# const withBundleAnalyzer = require('@next/bundle-analyzer')({ enabled: process.env.ANALYZE === 'true' });

Vite/Nuxt:

npx vite-bundle-visualizer
# Or
npx nuxi analyze

Generic webpack:

npx webpack-bundle-analyzer stats.json

Source map explorer:

npx source-map-explorer dist/**/*.js

Step 5: Profile Runtime Performance

Chrome DevTools Performance tab:

1. Open DevTools → Performance
2. Click Record
3. Perform user actions
4. Stop recording
5. Analyze flame chart for long tasks

Measure in code:

// Performance marks
performance.mark("start-operation");
await heavyOperation();
performance.mark("end-operation");
performance.measure("operation", "start-operation", "end-operation");

const measures = performance.getEntriesByName("operation");
console.log(`Operation took ${measures[0].duration}ms`);

React Profiler:

import { Profiler } from "react";

<Profiler
  id="Component"
  onRender={(id, phase, actualDuration) => {
    console.log(`${id} ${phase} render: ${actualDuration}ms`);
  }}
>
  <Component />
</Profiler>;

Step 6: Generate Recommendations

Report template:

## Performance Audit Report

**URL**: https://example.com
**Date**: 2026-01-18
**Device**: Mobile (Moto G Power)

### Scores

| Category       | Score |
| -------------- | ----- |
| Performance    | 72    |
| Accessibility  | 95    |
| Best Practices | 92    |
| SEO            | 100   |

### Core Web Vitals

| Metric | Value | Status               |
| ------ | ----- | -------------------- |
| LCP    | 3.2s  | ⚠️ Needs Improvement |
| INP    | 150ms | ✅ Good              |
| CLS    | 0.05  | ✅ Good              |

### Top Issues

#### 1. Large Contentful Paint (3.2s)

**Impact**: High
**Cause**: Hero image not optimized
**Fix**:

- Add `priority` to Next.js Image
- Use responsive srcset
- Preload LCP image

#### 2. Render-blocking resources

**Impact**: Medium
**Cause**: 3 CSS files blocking render
**Fix**:

- Inline critical CSS
- Defer non-critical styles
- Use `media` attribute for print styles

#### 3. Unused JavaScript (245KB)

**Impact**: Medium
**Cause**: Large vendor bundle
**Fix**:

- Enable tree shaking
- Dynamic import heavy components
- Remove unused dependencies

Common Optimizations

Images

// Next.js - Use Image component with priority for LCP
import Image from "next/image";

<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority // Preloads LCP image
  sizes="(max-width: 768px) 100vw, 1200px"
/>;

Code Splitting

// React - Lazy load heavy components
import { lazy, Suspense } from "react";

const HeavyChart = lazy(() => import("./HeavyChart"));

<Suspense fallback={<Loading />}>
  <HeavyChart />
</Suspense>;

// Dynamic import for conditional features
const module = await import("./heavy-module");

Font Optimization

// Next.js - Use next/font
import { Inter } from "next/font/google";

const inter = Inter({ subsets: ["latin"], display: "swap" });

/* Self-hosted with font-display */
@font-face {
  font-family: "Custom";
  src: url("/fonts/custom.woff2") format("woff2");
  font-display: swap;
}

Preloading Critical Resources

<!-- Preload LCP image -->
<link rel="preload" as="image" href="/hero.webp" />

<!-- Preconnect to CDN -->
<link rel="preconnect" href="https://cdn.example.com" />

<!-- DNS prefetch for third parties -->
<link rel="dns-prefetch" href="https://analytics.example.com" />

Reducing Layout Shift

/* Reserve space for images */
img {
  aspect-ratio: 16 / 9;
  width: 100%;
  height: auto;
}

/* Reserve space for ads/embeds */
.ad-slot {
  min-height: 250px;
}

// Always provide width/height
<Image width={800} height={450} ... />

Caching

// next.config.js - Cache static assets
module.exports = {
  async headers() {
    return [
      {
        source: "/_next/static/:path*",
        headers: [
          {
            key: "Cache-Control",
            value: "public, max-age=31536000, immutable",
          },
        ],
      },
    ];
  },
};

Validation

Before completing:

• Lighthouse performance score ≥90
• All Core Web Vitals in "Good" range
• No render-blocking resources
• Bundle size within budget
• No layout shifts from loading content

Error Handling

• Lighthouse fails to run: Ensure Chrome is installed and no conflicting flags.
• Metrics vary wildly: Run multiple times and average; use --throttling.cpuSlowdownMultiplier.
• Can't reproduce field data: Lab data differs from real users; use CrUX for field data.
• Build analysis fails: Ensure source maps are generated (devtool: 'source-map').

Resources

• web.dev Performance
• Lighthouse Documentation
• Core Web Vitals
• Chrome UX Report