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webperf-loading

Intelligent loading performance analysis with automated workflows for TTFB investigation (DNS/connection/server breakdown), render-blocking detection, script performance deep dive (first vs third-party attribution), font optimization, and resource hints validation. Includes decision trees that automatically analyze TTFB sub-parts when slow, detect script loading anti-patterns (async/defer/preload conflicts), identify render-blocking resources, and validate resource hints usage. Features workflows for complete loading audit (6 phases), backend performance investigation, and priority optimization. Cross-skill integration with Core Web Vitals (LCP resource loading), Interaction (script execution blocking), and Media (lazy loading strategy). Use when the user asks about TTFB, FCP, render-blocking, slow loading, font performance, script optimization, or resource hints. Compatible with Chrome DevTools MCP.

dist/antigravity/webperf-loading/SKILL.md

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Nov 6, 2026

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Nov 7, 2026
License
MIT

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# WebPerf: Loading Performance

JavaScript snippets for measuring web performance in Chrome DevTools.

## Running a script

1. Run the script with `mcp__chrome-devtools__evaluate_script`, passing the file instead of its code: `sourcePath` with the absolute path of `scripts/<Name>.js` in this skill, and `format: "script"`, since a script runs by itself and is not a function. Read the object it returns. The scripts have no console output, so the returned object is the result: `script`, `status` and the metric data (`value`, `rating`, `details`, `items`, `issues`), as described in `references/schema.md`.
2. When `status` is `"tracking"`, the script keeps observing the page. After the interactions, call the function named in `getDataFn` with another `mcp__chrome-devtools__evaluate_script` call, as an arrow function in `function`, for example `() => getINP()`, and read the object it returns.
3. When `status` is `"error"` or `"unsupported"`, read `error`: the page may not have loaded yet, or the browser lacks the API.

`sourcePath` and `format` need Chrome DevTools MCP 1.11.0 or later, and the server only reads files inside its workspace roots: the project, the temp folder and each `--workspace=<folder>` in its arguments. When it answers `Access denied`, as it does for a skill installed outside the project, such as in `~/.claude/skills/`, read the file and pass its code in `function`, still with `format: "script"`. A `--workspace` with the real folder of the skills makes the path work; the server follows symlinks, so a link to the folder is not enough.

Without Chrome DevTools MCP, run the same snippet with the [webperf-snippets CLI](https://webperf-snippets.net/CLI), which loads the page in headless Chromium and prints the same result objects: `npx webperf-snippets <url> --snippet <Name> --json`, where `<Name>` is the script name without `.js`. It needs Playwright and Chromium (`npm install --save-dev webperf-snippets playwright`, then `npx playwright install chromium`), and tracking snippets take `--interact-script <file>` to drive the interactions.