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:

Open DevTools → Performance
Click Record
Perform user actions
Stop recording
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').

profiling-performance