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chrome-audit-performance

Why It Exists​

Like its siblings, this skill began life as a Codex/VS Code prompt, performance-snapshot.prompt.md, added in the bulk baseline commit c0ba2ee2 ("prompts", 2025-12-03), described there as collecting "client-side performance metrics and largest resource contributors." It was converted to Claude Skill format in 44d082a3 ("chore: add skills for prompts") and moved to agent-skills/skills/chrome-audit-performance/ in 856e34fa. A later commit, 29619062 ("feat(skills): add frontend quality workflows"), touched the broader skills tree and references this skill as part of a frontend-quality pipeline, but does not change its content in a way that reveals distinct rationale. It was ultimately dropped from dotfiles in df4241d4 when the whole chrome-* family was migrated to the external catesandrew/skills marketplace. No unique backstory beyond "fast baseline without full Lighthouse overhead" — that's stated plainly in the SKILL.md itself.

What It Does​

The skill navigates to a URL with mcp__chrome__navigate_page and waits for the load event (or roughly 3 seconds), then runs a single evaluate_script call that pulls from three Performance API surfaces at once: performance.getEntriesByType('navigation') for TTFB/DCL/load timings and transfer size/protocol, performance.getEntriesByType('paint') for First Paint and First Contentful Paint, and performance.getEntriesByType('largest-contentful-paint') for LCP time and the LCP element's tag name. It also sorts performance.getEntriesByType('resource') by transfer size and keeps the top 10 heaviest resources.

The report renders navigation timing (TTFB, DCL, Load, transfer size, protocol), a Core Web Vitals table with FP/FCP/LCP values, and the top-10 heaviest resources — plus an optional screenshot. A fixed thresholds table classifies FCP, LCP, TTFB, and INP into Good/Needs Improvement/Poor bands per the standard web-vitals cutoffs (e.g., LCP < 2.5s good, 2.5–4s needs improvement, > 4s poor).

It's explicit about what it is not: these are synthetic, unthrottled metrics, not field data, so results should not be treated as equivalent to a throttled Lighthouse run or real-world CrUX data. It also notes that INP requires a PerformanceObserver on the event type during an actual interaction — it won't show up via getEntriesByType, so this skill only reports it as unavailable/null unless a caller separately wires that observer in.

How To Use It​

Triggers on: "client-side performance metrics from a running page via Chrome DevTools MCP", "navigation timing, Core Web Vitals (FCP, LCP), and largest resource contributors", wanting a fast baseline without the overhead of a full Lighthouse run.

skills add -g catesandrew/skills --skill skills/chrome-audit-performance
npm install @catesworks/skill-chrome-audit-performance
/plugin marketplace add catesandrew/skills
/plugin install cw@skills

This last one installs all 52 skills as a single bundled plugin named cw — there is no standalone per-skill plugin.

Gotchas & Invariants​

  • Requires a running, connected Chrome DevTools MCP server.
  • Measuring a cached page gives misleading cold-load numbers — hard-reload (Ctrl+Shift+R) before measuring.
  • LCP only finalizes after the load event fires, so cutting the wait short can under-report it as missing.
  • Don't conflate DCL (HTML parsed) with Load (everything downloaded) — they measure different milestones.
  • These are synthetic, unthrottled metrics — for real-world estimates, run Lighthouse with throttling or use field data (CrUX) instead.
  • transferSize = 0 means cache-served; it doesn't affect FCP/LCP timing but will skew size totals if misread as "missing."
  • chrome-audit-network — shares the same Performance API foundation, focused on per-request breakdown rather than page-level vitals.
  • chrome-audit-bundles — complements the heaviest-resources table with deeper bundle composition analysis.
  • chrome-audit-css — another audit-family sibling useful for tracing render-blocking CSS contributing to poor LCP/FCP.

Sourced from: skills/chrome-audit-performance/SKILL.md, skills/chrome-audit-performance/metadata.json, ~/.dotfiles git history (commit 44d082a3)