webperf-snippets

WebPerf Snippets

A curated collection of JavaScript snippets to measure and debug Web Performance directly in your browser's DevTools console.

Start here

• Measure your first page: start with LCP, CLS, and INP.
• Investigate slow loading: continue with TTFB, FCP, and render-blocking resources.
• Debug interaction issues: use Long Animation Frames and Scroll Performance.
• Audit media-heavy pages: open Image Element Audit and Video Element Audit.
• Automate the workflow with agents: see SKILLS.md and the installation section below.

What you can measure

Category What it includes Core Web Vitals LCP, CLS, INP - the metrics that impact SEO and user experience Loading TTFB, resource hints, scripts, fonts, images, render-blocking resources Interaction Long Animation Frames, event timing, responsiveness

How to use

Option 1: Run in browser console

1. Copy any snippet from webperf-snippets.nucliweb.net
2. Open DevTools (F12 or Cmd+Option+I / Ctrl+Shift+I)
3. Go to the Console tab
4. Paste and press Enter

Option 2: Save as DevTools Snippet

Save frequently used snippets for quick access:

1. Open DevTools → Sources tab → Snippets panel
2. Click + New snippet
3. Name it (e.g., "LCP")
4. Paste the code
5. Right-click → Run (or Cmd+Enter / Ctrl+Enter)

Video tutorial

https://github.com/nucliweb/webperf-snippets/assets/1307927/2987a2ca-3eef-4b73-8f6b-7b1e06b50040

Agent Skills for AI Coding Assistants

WebPerf Snippets can be used as Agent Skills with AI coding assistants like Claude Code, Cursor, OpenCode, Gemini CLI, and more for automated performance analysis.

Installation

Option 1: Using skills CLI (recommended)

npx skills add nucliweb/webperf-snippets

Installs directly from GitHub repository to ~/.claude/skills/.

Option 2: Install locally (project-specific)

git clone https://github.com/nucliweb/webperf-snippets.git
cd webperf-snippets
npm run install-skills

This installs skills to .claude/skills/ in your project directory.

Option 3: Install globally (for contributors)

git clone https://github.com/nucliweb/webperf-snippets.git
cd webperf-snippets
npm run install-global

This installs skills to ~/.claude/skills/ for use across any project.

Available Skills

Skill Snippets Description webperf 49 Main entry point for all web performance analysis webperf-core-web-vitals 7 LCP, CLS, INP measurements with detailed breakdowns webperf-loading 29 TTFB, FCP, script/font analysis, resource hints, service workers webperf-interaction 8 Long tasks, animation frames, scroll jank, INP debugging webperf-media 3 Image/video audits, lazy loading validation, SVG analysis webperf-resources 1 Network bandwidth, connection quality, adaptive loading

Intelligent Workflows & Decision Trees

Skills include built-in workflows and decision trees that enable autonomous performance analysis:

Workflows - Predefined sequences for common scenarios:

• Complete loading audit (6 snippets)
• Server performance investigation (4 snippets)
• Font loading optimization (3 snippets)
• Script performance deep dive (6 snippets)
• And more...

Decision Trees - Conditional logic based on results:

• If TTFB > 600ms → automatically run TTFB-Sub-Parts.js
• If FCP > 1.8s → run render-blocking resource analysis
• If many third-party scripts detected → analyze timing and impact
• And 15+ more intelligent triggers

Example interaction:

User: "Analyze loading performance"

Agent:
1. Runs TTFB.js → detects 750ms (slow)
2. Follows decision tree → runs TTFB-Sub-Parts.js
3. Identifies DNS + server time issues
4. Runs Service-Worker-Analysis.js
5. Provides consolidated report with specific fixes

Usage

Skills activate automatically when matched:

Audit this page for Core Web Vitals
Analyze loading performance
Check for image optimization issues
Debug slow LCP
Why is my TTFB so high?

Skills are compatible with Chrome DevTools MCP for automated browser-based performance auditing.

Supported agents: Claude Code, Cursor, OpenCode, Gemini CLI, VS Code extensions, and many more.

CLI

The repository includes a headless CLI (cli/) that runs WebPerf Snippets via Playwright — useful for CI pipelines and performance budgeting without copy-pasting into DevTools.

npx webperf-snippets https://web.dev --budget-lcp 2500 --budget-cls 0.1

See cli/README.md for installation, usage, and CI integration. The CLI is published to npm as webperf-snippets via a tag-based release workflow — pushing a cli-v* tag triggers the publish pipeline.

Documentation

Visit webperf-snippets.nucliweb.net for the full documentation with all snippets.

Resources

• Web Vitals - Learn about Core Web Vitals
• Chrome DevTools - Official documentation

Star History