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Your knowledge of web performance metrics, thresholds, and tooling APIs may be outdated.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)
1---2name: web-performance-optimization-23description: Analyzes web performance using Chrome DevTools MCP. Measures Core Web Vitals (FCP, LCP, TBT, CLS, 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.4license: Apache-2.05---6
7# Web Performance Audit
8
9Your knowledge of web performance metrics, thresholds, and tooling APIs may be outdated. **Prefer retrieval over pre-training** when citing specific numbers or recommendations.
10
11## Retrieval Sources
12
13| Source | How to retrieve | Use for |
14|--------|----------------|---------|
15| web.dev | `https://web.dev/articles/vitals` | Core Web Vitals thresholds, definitions |
16| Chrome DevTools docs | `https://developer.chrome.com/docs/devtools/performance` | Tooling APIs, trace analysis |
17| Lighthouse scoring | `https://developer.chrome.com/docs/lighthouse/performance/performance-scoring` | Score weights, metric thresholds |
18
19## FIRST: Verify MCP Tools Available
20
21**Run this before starting.** Try calling `navigate_page` or `performance_start_trace`. If unavailable, STOP—the chrome-devtools MCP server isn't configured.
22
23Ask the user to add this to their MCP config:
24
25```json
26"chrome-devtools": {
27 "type": "local",
28 "command": ["npx", "-y", "chrome-devtools-mcp@latest"]
29}
30```
31
32## Key Guidelines
33
34- **Be assertive**: Verify claims by checking network requests, DOM, or codebase—then state findings definitively.
35- **Verify before recommending**: Confirm something is unused before suggesting removal.
36- **Quantify impact**: Use estimated savings from insights. Don't prioritize changes with 0ms impact.
37- **Skip non-issues**: If render-blocking resources have 0ms estimated impact, note but don't recommend action.
38- **Be specific**: Say "compress hero.png (450KB) to WebP" not "optimize images".
39- **Prioritize ruthlessly**: A site with 200ms LCP and 0 CLS is already excellent—say so.
40
41## Quick Reference
42
43| Task | Tool Call |
44|------|-----------|
45| Load page | `navigate_page(url: "...")` |
46| Start trace | `performance_start_trace(autoStop: true, reload: true)` |
47| Analyze insight | `performance_analyze_insight(insightSetId: "...", insightName: "...")` |
48| List requests | `list_network_requests(resourceTypes: ["Script", "Stylesheet", ...])` |
49| Request details | `get_network_request(reqid: <id>)` |
50| A11y snapshot | `take_snapshot(verbose: true)` |
51
52## Workflow
53
54Copy this checklist to track progress:
55
56```
57Audit Progress:
58- [ ] Phase 1: Performance trace (navigate + record)
59- [ ] Phase 2: Core Web Vitals analysis (includes CLS culprits)
60- [ ] Phase 3: Network analysis
61- [ ] Phase 4: Accessibility snapshot
62- [ ] Phase 5: Codebase analysis (skip if third-party site)
63```
64
65### Phase 1: Performance Trace
66
671. Navigate to the target URL:
68 ```
69 navigate_page(url: "<target-url>")
70 ```
71
722. Start a performance trace with reload to capture cold-load metrics:
73 ```
74 performance_start_trace(autoStop: true, reload: true)
75 ```
76
773. Wait for trace completion, then retrieve results.
78
79**Troubleshooting:**
80- If trace returns empty or fails, verify the page loaded correctly with `navigate_page` first
81- If insight names don't match, inspect the trace response to list available insights
82
83### Phase 2: Core Web Vitals Analysis
84
85Use `performance_analyze_insight` to extract key metrics.
86
87**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.
88
89Common insight names:
90
91| Metric | Insight Name | What to Look For |
92|--------|--------------|------------------|
93| LCP | `LCPBreakdown` | Time to largest contentful paint; breakdown of TTFB, resource load, render delay |
94| CLS | `CLSCulprits` | Elements causing layout shifts (images without dimensions, injected content, font swaps) |
95| Render Blocking | `RenderBlocking` | CSS/JS blocking first paint |
96| Document Latency | `DocumentLatency` | Server response time issues |
97| Network Dependencies | `NetworkRequestsDepGraph` | Request chains delaying critical resources |
98
99Example:
100```
101performance_analyze_insight(insightSetId: "<id-from-trace>", insightName: "LCPBreakdown")
102```
103
104**Key thresholds (good/needs-improvement/poor):**
105- TTFB: < 800ms / < 1.8s / > 1.8s
106- FCP: < 1.8s / < 3s / > 3s
107- LCP: < 2.5s / < 4s / > 4s
108- INP: < 200ms / < 500ms / > 500ms
109- TBT: < 200ms / < 600ms / > 600ms
110- CLS: < 0.1 / < 0.25 / > 0.25
111- Speed Index: < 3.4s / < 5.8s / > 5.8s
112
113### Phase 3: Network Analysis
114
115List all network requests to identify optimization opportunities:
116```
117list_network_requests(resourceTypes: ["Script", "Stylesheet", "Document", "Font", "Image"])
118```
119
120**Look for:**
121
1221. **Render-blocking resources**: JS/CSS in `<head>` without `async`/`defer`/`media` attributes
1232. **Network chains**: Resources discovered late because they depend on other resources loading first (e.g., CSS imports, JS-loaded fonts)
1243. **Missing preloads**: Critical resources (fonts, hero images, key scripts) not preloaded
1254. **Caching issues**: Missing or weak `Cache-Control`, `ETag`, or `Last-Modified` headers
1265. **Large payloads**: Uncompressed or oversized JS/CSS bundles
1276. **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.
128
129For detailed request info:
130```
131get_network_request(reqid: <id>)
132```
133
134### Phase 4: Accessibility Snapshot
135
136Take an accessibility tree snapshot:
137```
138take_snapshot(verbose: true)
139```
140
141**Flag high-level gaps:**
142- Missing or duplicate ARIA IDs
143- Elements with poor contrast ratios (check against WCAG AA: 4.5:1 for normal text, 3:1 for large text)
144- Focus traps or missing focus indicators
145- Interactive elements without accessible names
146
147## Phase 5: Codebase Analysis
148
149**Skip if auditing a third-party site without codebase access.**
150
151Analyze the codebase to understand where improvements can be made.
152
153### Detect Framework & Bundler
154
155Search for configuration files to identify the stack:
156
157| Tool | Config Files |
158|------|--------------|
159| Webpack | `webpack.config.js`, `webpack.*.js` |
160| Vite | `vite.config.js`, `vite.config.ts` |
161| Rollup | `rollup.config.js`, `rollup.config.mjs` |
162| esbuild | `esbuild.config.js`, build scripts with `esbuild` |
163| Parcel | `.parcelrc`, `package.json` (parcel field) |
164| Next.js | `next.config.js`, `next.config.mjs` |
165| Nuxt | `nuxt.config.js`, `nuxt.config.ts` |
166| SvelteKit | `svelte.config.js` |
167| Astro | `astro.config.mjs` |
168
169Also check `package.json` for framework dependencies and build scripts.
170
171### Tree-Shaking & Dead Code
172
173- **Webpack**: Check for `mode: 'production'`, `sideEffects` in package.json, `usedExports` optimization
174- **Vite/Rollup**: Tree-shaking enabled by default; check for `treeshake` options
175- **Look for**: Barrel files (`index.js` re-exports), large utility libraries imported wholesale (lodash, moment)
176
177### Unused JS/CSS
178
179- Check for CSS-in-JS vs. static CSS extraction
180- Look for PurgeCSS/UnCSS configuration (Tailwind's `content` config)
181- Identify dynamic imports vs. eager loading
182
183### Polyfills
184
185- Check for `@babel/preset-env` targets and `useBuiltIns` setting
186- Look for `core-js` imports (often oversized)
187- Check `browserslist` config for overly broad targeting
188
189### Compression & Minification
190
191- Check for `terser`, `esbuild`, or `swc` minification
192- Look for gzip/brotli compression in build output or server config
193- Check for source maps in production builds (should be external or disabled)
194
195## Output Format
196
197Present findings as:
198
1991. **Core Web Vitals Summary** - Table with metric, value, and rating (good/needs-improvement/poor)
2002. **Top Issues** - Prioritized list of problems with estimated impact (high/medium/low)
2013. **Recommendations** - Specific, actionable fixes with code snippets or config changes
2024. **Codebase Findings** - Framework/bundler detected, optimization opportunities (omit if no codebase access)
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