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WebPerf Snippets

Tested JavaScript snippets that measure and debug web performance. Your AI agent runs them for you, or you run them yourself in DevTools.

For agents: Tested JavaScript snippets measuring Core Web Vitals (CLS, INP) and debugging web performance, runnable via CLI, MCP server or DevTools extension.

web performancecore web vitalsjavascript snippetsmcp serverdevtools

Their llms.txt

The beginning of the file, from our validated copy saved 2026-10-24T08:00:44.200Z. Cached copy · Source on webperf-snippets.net

# WebPerf Snippets

> Tested JavaScript snippets that measure and debug web performance. Your AI agent runs them for you, or you run them yourself in DevTools.
> Source: https://webperf-snippets.net
> Version: 2.0.1
> Each link points to the Markdown version of a page; the HTML page is at the same URL without .md.

---

## Guides

- [WebPerf Snippets CLI](https://webperf-snippets.net/CLI.md)
- [MCP server](https://webperf-snippets.net/MCP.md)
- [DevTools extension](https://webperf-snippets.net/extension.md)
- [Which snippet should I use?](https://webperf-snippets.net/which-snippet.md)

## Core Web Vitals

- [Cumulative layout shift (CLS)](https://webperf-snippets.net/CoreWebVitals/CLS.md)
  Quick check for Cumulative Layout Shift, a Core Web Vital that measures visual stability. CLS tracks how much the page layout shifts unexpectedly during its lifetime, providing a single score that represents the largest burst of unexpected layout shifts.

- [Interaction to next paint (INP)](https://webperf-snippets.net/CoreWebVitals/INP.md)
  Tracks Interaction to Next Paint, a Core Web Vital that measures responsiveness. INP evaluates how quickly a page responds to user interactions throughout the entire page visit, replacing First Input Delay (FID) as a Core Web Vital in March 2024.

- [LCP image entropy](https://webperf-snippets.net/CoreWebVitals/LCP-Image-Entropy.md)
  Checks if images qualify as LCP candidates based on their entropy (bits per pixel). Since Chrome 112, low-entropy images are ignored for LCP measurement.

- [LCP subparts](https://webperf-snippets.net/CoreWebVitals/LCP-Subparts.md)
  Breaks down Largest Contentful Paint into its four phases to identify optimization opportunities. Understanding which phase is slowest helps you focus your optimization efforts where they'll have the most impact. Based on the Web Vitals Chrome Extension.

- [LCP trail](https://webperf-snippets.net/CoreWebVitals/LCP-Trail.md)
  Tracks every LCP candidate element during page load and highlights each one with a distinct pastel-colored dashed outline, so you can see the full trail from first candidate to final LCP.

- [LCP video candidate](https://webperf-snippets.net/CoreWebVitals/LCP-Video-Candidate.md)
  Detects whether the LCP element is a <video> and audits the poster image configuration, the most common source of avoidable LCP delay when video is the hero element.

- [Largest contentful paint (LCP)](https://webperf-snippets.net/CoreWebVitals/LCP.md)
  Quick check for Largest Contentful Paint, a Core Web Vital that measures loading performance. LCP marks when the largest content element becomes visible in the viewport.

## Loading

- [Back/forward cache (bfcache)](https://webperf-snippets.net/Loading/Back-Forward-Cache.md)
  Analyzes Back/Forward Cache (bfcache) to determine if your page is eligible for instant back/forward navigations. When bfcache works, users get instant (0ms) page loads when using browser back/forward buttons, dramatically improving perceiv
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