Web Perf · cloudflare/skills · Skills Docs

Skills

Chapter 11 of 13

Web Perf

Analyzes web performance using Chrome DevTools MCP.

4 minutes · 772 words · 17 sections

Web Performance Audit

Your knowledge of web performance metrics, thresholds, and tooling APIs may be outdated. Prefer retrieval over pre-training when citing specific numbers or recommendations.

Retrieval Sources

Source	How to retrieve	Use for
web.dev	`https://web.dev/articles/vitals`	Core Web Vitals thresholds, definitions
Chrome DevTools docs	`https://developer.chrome.com/docs/devtools/performance`	Tooling APIs, trace analysis
Lighthouse scoring	`https://developer.chrome.com/docs/lighthouse/performance/performance-scoring`	Score weights, metric thresholds

FIRST: Verify MCP Tools Available

Run this before starting. Try calling navigate_page or performance_start_trace. If unavailable, STOP—the chrome-devtools MCP server isn’t configured.

Ask the user to add this to their MCP config:

"chrome-devtools": {
  "type": "local",
  "command": ["npx", "-y", "chrome-devtools-mcp@latest"]
}

Key Guidelines

Be assertive: Verify claims by checking network requests, DOM, or codebase—then state findings definitively.
Verify before recommending: Confirm something is unused before suggesting removal.
Quantify impact: Use estimated savings from insights. Don’t prioritize changes with 0ms impact.
Skip non-issues: If render-blocking resources have 0ms estimated impact, note but don’t recommend action.
Be specific: Say “compress hero.png (450KB) to WebP” not “optimize images”.
Prioritize ruthlessly: A site with 200ms LCP and 0 CLS is already excellent—say so.

Quick Reference

Task	Tool Call
Load page	`navigate_page(url: "...")`
Start trace	`performance_start_trace(autoStop: true, reload: true)`
Analyze insight	`performance_analyze_insight(insightSetId: "...", insightName: "...")`
List requests	`list_network_requests(resourceTypes: ["Script", "Stylesheet", ...])`
Request details	`get_network_request(reqid: <id>)`
A11y snapshot	`take_snapshot(verbose: true)`

Workflow

Copy this checklist to track progress:

Audit Progress:
- [ ] Phase 1: Performance trace (navigate + record)
- [ ] Phase 2: Core Web Vitals analysis (includes CLS culprits)
- [ ] Phase 3: Network analysis
- [ ] Phase 4: Accessibility snapshot
- [ ] Phase 5: Codebase analysis (skip if third-party site)

Phase 1: Performance Trace

Navigate to the target URL:
```
navigate_page(url: "<target-url>")
```
Start a performance trace with reload to capture cold-load metrics:
```
performance_start_trace(autoStop: true, reload: true)
```
Wait for trace completion, then retrieve results.

Troubleshooting:

If trace returns empty or fails, verify the page loaded correctly with navigate_page first
If insight names don’t match, inspect the trace response to list available insights

Phase 2: Core Web Vitals Analysis

Use performance_analyze_insight to extract key metrics.

Note: Insight names may vary across Chrome DevTools versions. If an insight name doesn’t work, check the insightSetId from the trace response to discover available insights.

Common insight names:

Metric	Insight Name	What to Look For
LCP	`LCPBreakdown`	Time to largest contentful paint; breakdown of TTFB, resource load, render delay
CLS	`CLSCulprits`	Elements causing layout shifts (images without dimensions, injected content, font swaps)
Render Blocking	`RenderBlocking`	CSS/JS blocking first paint
Document Latency	`DocumentLatency`	Server response time issues
Network Dependencies	`NetworkRequestsDepGraph`	Request chains delaying critical resources

Example:

performance_analyze_insight(insightSetId: "<id-from-trace>", insightName: "LCPBreakdown")

Key thresholds (good/needs-improvement/poor):

TTFB: < 800ms / < 1.8s / > 1.8s
FCP: < 1.8s / < 3s / > 3s
LCP: < 2.5s / < 4s / > 4s
INP: < 200ms / < 500ms / > 500ms
TBT: < 200ms / < 600ms / > 600ms
CLS: < 0.1 / < 0.25 / > 0.25
Speed Index: < 3.4s / < 5.8s / > 5.8s

Phase 3: Network Analysis

List all network requests to identify optimization opportunities:

list_network_requests(resourceTypes: ["Script", "Stylesheet", "Document", "Font", "Image"])

Look for:

Render-blocking resources: JS/CSS in <head> without async/defer/media attributes
Network chains: Resources discovered late because they depend on other resources loading first (e.g., CSS imports, JS-loaded fonts)
Missing preloads: Critical resources (fonts, hero images, key scripts) not preloaded
Caching issues: Missing or weak Cache-Control, ETag, or Last-Modified headers
Large payloads: Uncompressed or oversized JS/CSS bundles
Unused preconnects: If flagged, verify by checking if ANY requests went to that origin. If zero requests, it’s definitively unused—recommend removal. If requests exist but loaded late, the preconnect may still be valuable.

For detailed request info:

get_network_request(reqid: <id>)

Phase 4: Accessibility Snapshot

Take an accessibility tree snapshot:

take_snapshot(verbose: true)

Flag high-level gaps:

Missing or duplicate ARIA IDs
Elements with poor contrast ratios (check against WCAG AA: 4.5:1 for normal text, 3:1 for large text)
Focus traps or missing focus indicators
Interactive elements without accessible names

Phase 5: Codebase Analysis

Skip if auditing a third-party site without codebase access.

Analyze the codebase to understand where improvements can be made.

Detect Framework & Bundler

Search for configuration files to identify the stack:

Tool	Config Files
Webpack	`webpack.config.js`, `webpack.*.js`
Vite	`vite.config.js`, `vite.config.ts`
Rollup	`rollup.config.js`, `rollup.config.mjs`
esbuild	`esbuild.config.js`, build scripts with `esbuild`
Parcel	`.parcelrc`, `package.json` (parcel field)
Next.js	`next.config.js`, `next.config.mjs`
Nuxt	`nuxt.config.js`, `nuxt.config.ts`
SvelteKit	`svelte.config.js`
Astro	`astro.config.mjs`

Also check package.json for framework dependencies and build scripts.

Tree-Shaking & Dead Code

Webpack: Check for mode: 'production', sideEffects in package.json, usedExports optimization
Vite/Rollup: Tree-shaking enabled by default; check for treeshake options
Look for: Barrel files (index.js re-exports), large utility libraries imported wholesale (lodash, moment)

Unused JS/CSS

Check for CSS-in-JS vs. static CSS extraction
Look for PurgeCSS/UnCSS configuration (Tailwind’s content config)
Identify dynamic imports vs. eager loading

Polyfills

Check for @babel/preset-env targets and useBuiltIns setting
Look for core-js imports (often oversized)
Check browserslist config for overly broad targeting

Compression & Minification

Check for terser, esbuild, or swc minification
Look for gzip/brotli compression in build output or server config
Check for source maps in production builds (should be external or disabled)

Output Format

Present findings as:

Core Web Vitals Summary - Table with metric, value, and rating (good/needs-improvement/poor)
Top Issues - Prioritized list of problems with estimated impact (high/medium/low)
Recommendations - Specific, actionable fixes with code snippets or config changes
Codebase Findings - Framework/bundler detected, optimization opportunities (omit if no codebase access)

Install this repository

$ npx skills add cloudflare/skills

» /plugin marketplace add cloudflare/skills

Skills install per repository, not per chapter — the CLI has no documented per-skill form, so we do not print one.

About this skill

Trigger

Analyzes web performance using Chrome DevTools MCP. Measures Core Web Vitals (LCP, INP, CLS) and supplementary metrics (FCP, TBT, Speed Index), identifies render-blocking resources, network dependency chains, layout shifts, caching issues, and accessibility gaps. Use when asked to audit, profile, debug, or optimize page load performance, Lighthouse scores, or site speed. Biases towards retrieval from current documentation over pre-trained knowledge.

The verbatim description from this skill’s front matter — the string an agent matches on to decide whether to load it.

Colophon for this issue

Colophon

How this issue was made

Source

github.com/cloudflare/skills, branch main, last pushed 7 August 2026.

Licence

Apache-2.0 — the text of every chapter is reproduced unmodified, frontmatter included, under the upstream licence.

Discovery

13 chapters found by walking the repository tree for SKILL.md, not by matching a directory convention. One layout observed: skills/*/SKILL.md.

Issue colours

Resolved from a curated brand profile — hue 54°, chroma 0.172. Two accent tones are generated per issue and each is proven against its own ground before it ships: a single accent that passes AA on both light and dark paper is arithmetically impossible.

Heading repairs

1 repair applied to this chapter so the document has one h1 and no skipped levels:

Shifted “17 headings” from h1 to h2 so the chapter title is the only h1.

Spec compliance

Every chapter satisfies the Agent Skills specification’s frontmatter rules.

Images

Images inside a chapter come from the upstream repository. Where the author gave no alternative text we mark the image decorative rather than inventing a description — a plausible caption we made up is worse than none for the reader who depends on it.

Marketplace

A plugin manifest is published at .claude-plugin/marketplace.json by Cloudflare, declaring 1 plugin. It is read for editorial metadata only — never as the skill index, which is always the repository tree.

Signal

Install counts come from skills.sh. They measure downloads, not quality, and an unranked repository is not an unread one.

Agent surfaces

The whole issue is available as one markdown document at /cloudflare/skills.md, and each chapter at its own URL.

.md