WebPerf: Core Web Vitals

JavaScript snippets for measuring web performance in Chrome DevTools.

Running a script

1. Run the script with mcp__chrome-devtools__evaluate_script and 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, 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.

Without Chrome DevTools MCP, run the same snippet with the webperf-snippets 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.

Scripts

• scripts/CLS.js — Cumulative layout shift (CLS)
• scripts/INP.js — Interaction to next paint (INP)
• scripts/LCP-Image-Entropy.js — LCP image entropy
• scripts/LCP-Subparts.js — LCP subparts
• scripts/LCP-Trail.js — LCP trail
• scripts/LCP-Video-Candidate.js — LCP video candidate
• scripts/LCP.js — Largest contentful paint (LCP)

Common Workflows

Complete Core Web Vitals Audit

When the user asks for a comprehensive Core Web Vitals analysis or "audit CWV":

1. LCP.js - Measure Largest Contentful Paint
2. CLS.js - Measure Cumulative Layout Shift
3. INP.js - Measure Interaction to Next Paint
4. LCP-Subparts.js - Break down LCP timing phases
5. LCP-Trail.js - Track LCP candidate evolution

LCP Deep Dive

When LCP is slow or the user asks "debug LCP" or "why is LCP slow":

1. LCP.js - Establish baseline LCP value
2. LCP-Subparts.js - Break down into TTFB, resource load, render delay
3. LCP-Trail.js - Identify all LCP candidates and changes
4. LCP-Image-Entropy.js - Check if LCP image has visual complexity issues
5. LCP-Video-Candidate.js - Detect if LCP is a video (poster or video element)

CLS Investigation

When layout shifts are detected or the user asks "debug CLS" or "layout shift issues":

1. CLS.js - Measure overall CLS score
2. Layout-Shift-Loading-and-Interaction.js (pending — available in webperf-interaction skill)
3. Cross-reference with webperf-loading skill:
   • Find-Above-The-Fold-Lazy-Loaded-Images.js (lazy images causing shifts)
   • Fonts-Preloaded-Loaded-and-used-above-the-fold.js (font swap causing shifts)

INP Debugging

When interactions feel slow or the user asks "debug INP" or "slow interactions":

1. INP.js - Measure overall INP value; call getINP() after interactions, getINPDetails() for the 15 slowest interactions
2. Interactions.js (pending — available in webperf-interaction skill)
3. Input-Latency-Breakdown.js (pending — available in webperf-interaction skill)
4. Long-Animation-Frames.js (pending — available in webperf-interaction skill)
5. Long-Animation-Frames-Script-Attribution.js (pending — available in webperf-interaction skill)

Video as LCP Investigation

When LCP is a video element (detected by LCP-Video-Candidate.js):

1. LCP-Video-Candidate.js - Identify video as LCP candidate
2. Video-Element-Audit.js (from Media skill) - Audit video loading strategy
3. LCP-Subparts.js - Analyze video loading phases
4. Cross-reference with webperf-loading skill:
   • Resource-Hints-Validation.js (check for video preload)
   • Priority-Hints-Audit.js (check for fetchpriority on video)

Image as LCP Investigation

When LCP is an image (most common case):

1. LCP.js - Measure LCP timing
2. LCP-Subparts.js - Break down timing phases
3. LCP-Image-Entropy.js - Analyze image complexity
4. Cross-reference with webperf-media skill:
   • Image-Element-Audit.js (check format, dimensions, lazy loading)
5. Cross-reference with webperf-loading skill:
   • Find-Above-The-Fold-Lazy-Loaded-Images.js (check if incorrectly lazy)
   • Priority-Hints-Audit.js (check for fetchpriority="high")
   • Resource-Hints-Validation.js (check for preload)

Decision Tree

Use this decision tree to automatically run follow-up snippets based on results:

After LCP.js

• If LCP > 2.5s → Run LCP-Subparts.js to diagnose which phase is slow
• If LCP > 4.0s (poor) → Run full LCP deep dive workflow (5 snippets)
• If LCP candidate is an image → Run LCP-Image-Entropy.js and webperf-media:Image-Element-Audit.js
• If LCP candidate is a video → Run LCP-Video-Candidate.js and webperf-media:Video-Element-Audit.js
• Always run → LCP-Trail.js to understand candidate evolution

After LCP-Subparts.js

• If TTFB phase > 600ms → Switch to webperf-loading skill and run TTFB-Sub-Parts.js
• If Resource Load Time > 1500ms → Run:
  1. webperf-loading:Resource-Hints-Validation.js (check for preload/preconnect)
  2. webperf-loading:Priority-Hints-Audit.js (check fetchpriority)
  3. webperf-loading:Find-render-blocking-resources.js (competing resources)
• If Render Delay > 200ms → Run:
  1. webperf-loading:Find-render-blocking-resources.js (blocking CSS/JS)
  2. webperf-loading:Script-Loading.js (parser-blocking scripts)
  3. webperf-interaction:Long-Animation-Frames.js (main thread blocking)

After LCP-Trail.js

• If many LCP candidate changes (>3) → This causes visual instability, investigate:
  1. webperf-loading:Find-Above-The-Fold-Lazy-Loaded-Images.js (late-loading images)
  2. webperf-loading:Fonts-Preloaded-Loaded-and-used-above-the-fold.js (font swaps)
  3. CLS.js (layout shifts contributing to LCP changes)
• If final LCP candidate appears late → Run webperf-loading:Resource-Hints-Validation.js
• If early candidate was replaced → Understand why initial content was pushed down (likely CLS issue)

After LCP-Image-Entropy.js

• If entropy is very high → Image is visually complex, recommend:
  • Modern formats (WebP, AVIF)
  • Appropriate compression
  • Potentially a placeholder strategy
• If entropy is low → Image may be over-optimized or placeholder-like
• If large file size detected → Run webperf-media:Image-Element-Audit.js for format/sizing analysis

After LCP-Video-Candidate.js

• If video is LCP → Run:
  1. webperf-media:Video-Element-Audit.js (check poster, preload, formats)
  2. webperf-loading:Priority-Hints-Audit.js (check fetchpriority on poster)
  3. LCP-Subparts.js (analyze video loading phases)
• If poster image is LCP → Treat as image LCP (run image workflows)

After CLS.js

• If CLS > 0.1 → Run webperf-interaction:Layout-Shift-Loading-and-Interaction.js (pending — available in webperf-interaction skill)
• If CLS > 0.25 (poor) → Run comprehensive shift investigation:
  1. webperf-loading:Find-Above-The-Fold-Lazy-Loaded-Images.js (images without dimensions)
  2. webperf-loading:Fonts-Preloaded-Loaded-and-used-above-the-fold.js (font loading strategy)
  3. webperf-loading:Critical-CSS-Detection.js (late-loading styles)
  4. webperf-media:Image-Element-Audit.js (missing width/height)
• If CLS = 0 → Confirm with multiple page loads (might be timing-dependent)

After INP.js

• If INP > 200ms → Run webperf-interaction:Interactions.js to identify slow interactions
• If INP > 500ms (poor) → Run full INP debugging workflow:
  1. webperf-interaction:Interactions.js (list all interactions)
  2. webperf-interaction:Input-Latency-Breakdown.js (phase breakdown)
  3. webperf-interaction:Long-Animation-Frames.js (blocking frames)
  4. webperf-interaction:Long-Animation-Frames-Script-Attribution.js (culprit scripts)
• If specific interaction type is slow (e.g., keyboard) → Focus analysis on that interaction type
• After INP data is collected → call getINPDetails() for the 15 slowest interactions, slowest first (useful for identifying patterns across multiple slow interactions)

Error Recovery

• If any script returns status: "error" → Check if the page has finished loading:
  • If early in load: wait and re-run the script
  • If page is an SPA: user may need to navigate to the target route first
• If LCP.js / LCP-Subparts.js returns status: "error" → Tell the user: "LCP data is not available yet. Please ensure the page has fully loaded, then run the analysis again."
• If INP.js getINP() returns status: "error" → The getDataFn: "getINP" field signals the agent can retry after user interaction. Prompt the user to click, type, or scroll, then call getINP() again.

Cross-Skill Triggers

Context fork note: This skill runs with context: fork. Cross-skill triggers below are recommendations to report back to the parent agent, not direct calls this subagent can execute. When a cross-skill trigger fires, tell the user which skill and script to run next. Scripts marked (pending) are not yet available — skip them and note the limitation.

These triggers recommend using snippets from other skills:

From LCP to Loading Skill

• If LCP > 2.5s and TTFB phase is dominant → Use webperf-loading skill:

  • TTFB.js, TTFB-Sub-Parts.js, Service-Worker-Analysis.js

• If LCP image is lazy-loaded → Use webperf-loading skill:

  • Find-Above-The-Fold-Lazy-Loaded-Images.js

• If LCP has no fetchpriority → Use webperf-loading skill:

  • Priority-Hints-Audit.js

From CLS to Loading Skill

• If CLS caused by fonts → Use webperf-loading skill:

  • Fonts-Preloaded-Loaded-and-used-above-the-fold.js
  • Resource-Hints-Validation.js (for font preload)

• If CLS caused by images → Use webperf-media skill:

  • Image-Element-Audit.js (check for width/height attributes)

From INP to Interaction Skill

• If INP > 200ms → Use webperf-interaction skill for full debugging:
  • Interactions.js, Input-Latency-Breakdown.js
  • Long-Animation-Frames.js, Long-Animation-Frames-Script-Attribution.js
  • LongTask.js (if pre-interaction blocking suspected)

From LCP/INP to Interaction Skill

• If render delay or interaction delay is high → Use webperf-interaction skill:
  • Long-Animation-Frames.js (main thread blocking)
  • LongTask.js (long tasks delaying rendering)

Multi-Metric Correlation

When multiple CWV metrics are poor, prioritize investigation:

• If LCP > 2.5s AND CLS > 0.1 → Likely shared root cause:

  1. Check for late-loading content pushing LCP element
  2. Run LCP-Trail.js to see LCP candidate changes
  3. Run Layout-Shift-Loading-and-Interaction.js to correlate timing

• If LCP > 2.5s AND INP > 200ms → Main thread congestion:

  1. Run Long-Animation-Frames.js
  2. Run webperf-loading:Script-Loading.js
  3. Run webperf-loading:JS-Execution-Time-Breakdown.js

• If CLS > 0.1 AND INP > 200ms → Layout thrashing or interaction-triggered shifts:

  1. Run Layout-Shift-Loading-and-Interaction.js
  2. Run Interactions.js
  3. Check if shifts occur during/after interactions

References

• references/snippets.md — Descriptions and thresholds for each script
• references/schema.md — Return value schema for interpreting script output