Webperf Interaction Agent Skill

WebPerf: Interaction & Animation

JavaScript snippets for measuring web performance in Chrome DevTools.

Running a script

• 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.
• 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.
• 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/DOM-Size-and-Depth.js — DOM size and depth
• scripts/Forced-Synchronous-Layout.js — Forced synchronous layout detector
• scripts/Input-Latency-Breakdown.js — Input latency breakdown
• scripts/Interactions.js — Interactions
• scripts/Layout-Shift-Loading-and-Interaction.js — Layout shift tracking
• scripts/Long-Animation-Frames-Helpers.js — LoAF helpers
• scripts/Long-Animation-Frames-Script-Attribution.js — Long animation frames script attribution
• scripts/Long-Animation-Frames.js — Long animation frames (LoAF)
• scripts/LongTask.js — Long tasks
• scripts/Scroll-Performance.js — Scroll performance analysis

Common Workflows

Complete Interaction Performance Audit

When the user asks for interaction analysis or "audit responsiveness":

• Interactions.js - List all user interactions with timing
• Input-Latency-Breakdown.js - Break down interaction phases
• Long-Animation-Frames.js - Detect blocking animation frames
• LongTask.js - Identify long tasks blocking the main thread
• Scroll-Performance.js - Measure scroll smoothness

INP Deep Dive

When INP is slow (>200ms) or the user asks "debug INP" or "slow interactions":

• Interactions.js - Identify which interactions are slow
• Input-Latency-Breakdown.js - Break down into input delay, processing time, presentation delay
• Long-Animation-Frames.js - Find animation frames >50ms that block interactions
• Long-Animation-Frames-Script-Attribution.js - Identify scripts causing the blocking
• Long-Animation-Frames-Helpers.js - Get detailed timing and attribution helpers

Scroll Jank Investigation

When the user reports "scroll jank", "choppy scrolling", or "scroll performance issues":

• Scroll-Performance.js - Measure scroll smoothness and frame drops
• Long-Animation-Frames.js - Detect frames causing jank (>50ms)
• Layout-Shift-Loading-and-Interaction.js - Check for layout shifts during scroll
• Long-Animation-Frames-Script-Attribution.js - Find scripts running during scroll

Main Thread Blocking Analysis

When the page feels unresponsive or the user asks "why is the page frozen":

• LongTask.js - List all tasks >50ms blocking the main thread
• Long-Animation-Frames.js - Correlate with animation frame timing
• Long-Animation-Frames-Script-Attribution.js - Attribute blocking to specific scripts
• Cross-reference with webperf-loading skill:
• JS-Execution-Time-Breakdown.js (script parsing/execution time)
• First-And-Third-Party-Script-Info.js (identify third-party blockers)

Layout Shift During Interaction

When the user reports "things move when I click" or CLS issues during interaction:

• Layout-Shift-Loading-and-Interaction.js - Separate loading vs interaction shifts
• Interactions.js - Correlate shifts with specific interactions
• CLS.js (from Core Web Vitals skill) - Measure total CLS
• Cross-reference with webperf-media skill:
• Image-Element-Audit.js (images without dimensions causing shifts)

Animation Performance Analysis

When animations feel sluggish or the user asks "debug animation performance":

• Long-Animation-Frames.js - Identify frames taking too long to render
• Long-Animation-Frames-Helpers.js - Detailed frame timing analysis
• Long-Animation-Frames-Script-Attribution.js - Find scripts delaying frames
• Scroll-Performance.js - If scroll animations are involved
• DOM-Size-and-Depth.js - Check whether a large or deep DOM makes each style and layout pass expensive

Third-Party Script Impact on Interactions

When interactions are slow and third-party scripts are suspected:

• Long-Animation-Frames-Script-Attribution.js - Identify third-party scripts in long frames
• LongTask.js - Measure long task frequency
• Cross-reference with webperf-loading skill:
• First-And-Third-Party-Script-Info.js (list all third-party scripts)
• First-And-Third-Party-Script-Timings.js (measure script loading impact)
• Script-Loading.js (check for blocking scripts)

Decision Tree

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

After Interactions.js

• If any interaction > 200ms → Run Input-Latency-Breakdown.js on slow interactions; also run webperf-core-web-vitals:INP.js for official INP measurement
• If many interactions > 200ms → Main thread congestion, run:
• Long-Animation-Frames.js (blocking frames)
• LongTask.js (long tasks)
• Long-Animation-Frames-Script-Attribution.js (culprit scripts)
• If specific interaction types are slow (e.g., click vs keyboard) → Focus on that interaction type
• If interactions occur during page load → Cross-check with webperf-loading:JS-Execution-Time-Breakdown.js

After Input-Latency-Breakdown.js

• If Input Delay > 50ms → Main thread was busy, run:
• Long-Animation-Frames.js (what was blocking)
• LongTask.js (long task before interaction)
• If Processing Time > 100ms → Heavy event handlers, run:
• Long-Animation-Frames-Script-Attribution.js (which scripts)
• webperf-loading:First-And-Third-Party-Script-Info.js (third-party involvement)
• If Presentation Delay > 50ms → Rendering bottleneck, run:
• Long-Animation-Frames.js (frame rendering time)
• Layout-Shift-Loading-and-Interaction.js (check for layout work)

After Long-Animation-Frames.js

• If many frames > 50ms → Run Long-Animation-Frames-Script-Attribution.js to identify causes
• If frames > 100ms → Critical blocking, run:
• Long-Animation-Frames-Script-Attribution.js (detailed attribution)
• LongTask.js (related long tasks)
• webperf-loading:JS-Execution-Time-Breakdown.js (script execution cost)
• If frames occur during scroll → Run Scroll-Performance.js to measure impact
• If frames occur during page load → Cross-check with webperf-loading:Event-Processing-Time.js

After Long-Animation-Frames-Script-Attribution.js

• If third-party scripts are causing long frames → Run:
• webperf-loading:First-And-Third-Party-Script-Info.js (list all third-party scripts)
• webperf-loading:Script-Loading.js (check loading strategy)
• Recommend deferring or removing problematic scripts
• If first-party scripts are causing long frames → Recommend:
• Code splitting
• Debouncing/throttling event handlers
• Web Workers for heavy computation
• If forced reflow/layout detected → Analyze DOM manipulation patterns
• If time goes to style recalculation or layout → Run DOM-Size-and-Depth.js to check the size of the tree

After LongTask.js

• If many long tasks (>5) → Main thread is congested, run:
• Long-Animation-Frames-Script-Attribution.js (detailed attribution)
• webperf-loading:JS-Execution-Time-Breakdown.js (script execution analysis)
• webperf-loading:First-And-Third-Party-Script-Info.js (identify heavy scripts)
• If long tasks > 500ms → Critical blocking, investigate:
• Synchronous third-party scripts
• Heavy computation without Web Workers
• Excessive DOM manipulation
• If long tasks correlate with interactions → Run Interactions.js to see impact

After DOM-Size-and-Depth.js

• If total elements > 1400 → Recommend virtualizing long lists and loading off-screen content on demand
• If depth > 32 → Recommend flattening wrapper elements added by components
• If a single parent has > 60 children → Recommend pagination or virtualization for that container
• If the DOM is within limits but interactions are still slow → Run Long-Animation-Frames-Script-Attribution.js to find the scripts responsible

After Scroll-Performance.js

• If scroll FPS < 30 → Severe jank, run:
• Long-Animation-Frames.js (blocking frames during scroll)
• Long-Animation-Frames-Script-Attribution.js (scripts running on scroll)
• Layout-Shift-Loading-and-Interaction.js (layout work during scroll)
• If scroll FPS 30-50 → Noticeable jank, investigate:
• Scroll event handlers
• Passive event listeners
• CSS will-change properties
• If scroll FPS > 50 → Good, but check for layout shifts during scroll

After Layout-Shift-Loading-and-Interaction.js

• If shifts occur during page load → Cross-reference with webperf-core-web-vitals:CLS.js
• If shifts occur during interaction → This impacts INP and UX, investigate:
• Dynamic content insertion
• Images/ads loading without dimensions
• Font swaps (run webperf-loading:Fonts-Preloaded-Loaded-and-used-above-the-fold.js)
• If shifts occur during scroll → Run Scroll-Performance.js to measure impact

After Long-Animation-Frames-Helpers.js

This is a utility snippet, use results to:

• Understand frame timing in detail
• Identify precise script attribution
• Measure style/layout/paint phases
• No automatic follow-up, use data to inform next steps

Performance Budget Thresholds

Use these thresholds to automatically trigger follow-up analysis:

INP Thresholds:

• Good: ≤ 200ms → Monitor, no action needed
• Needs Improvement: 200-500ms → Run Input-Latency-Breakdown.js
• Poor: > 500ms → Run full INP debugging workflow (5 snippets)

Long Animation Frames:

• Warning: > 50ms → Run Long-Animation-Frames-Script-Attribution.js
• Critical: > 100ms → Run full main thread blocking workflow

Long Tasks:

• Warning: > 50ms → Monitor frequency
• Critical: > 200ms or >5 tasks per second → Run attribution and script analysis

Scroll Performance:

• Good: ≥ 50 FPS → Monitor
• Needs Improvement: 30-50 FPS → Run Long-Animation-Frames.js
• Poor: < 30 FPS → Run full scroll jank workflow

Interaction Frequency:

• If >10 interactions/second → User is actively interacting, prioritize INP optimization
• If <1 interaction/5 seconds → Interactions are rare, focus on first interaction responsiveness

Multi-Metric Correlation

When multiple interaction metrics are poor:

• If Long Tasks AND Long Animation Frames both detected → Shared root cause:
• Run Long-Animation-Frames-Script-Attribution.js
• Likely heavy JavaScript execution
• Consider code splitting or Web Workers

• If INP slow AND CLS high → Interaction-triggered layout shifts:
• Run Layout-Shift-Loading-and-Interaction.js
• Correlate shift timing with interaction timing
• Check for dynamic content insertion

• If Scroll jank AND Long Animation Frames → Scroll handlers blocking main thread:
• Run Long-Animation-Frames-Script-Attribution.js during scroll
• Check for non-passive scroll listeners
• Audit scroll-triggered animations

References

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