webperf-resources

WebPerf: Resources & Network

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/Network-Bandwidth-Connection-Quality.js — Network bandwidth & connection quality

Common Workflows

Network Quality Assessment

When the user asks about network performance, connection quality, or adaptive loading:

1. Network-Bandwidth-Connection-Quality.js - Analyze network bandwidth, effective connection type, RTT, downlink, save-data mode

Adaptive Loading Strategy

When implementing adaptive loading or the user asks "how to optimize for slow connections":

1. Network-Bandwidth-Connection-Quality.js - Detect connection quality
2. Cross-reference with webperf-loading skill:
   • TTFB.js (measure server response on slow connections)
   • Find-render-blocking-resources.js (identify heavy resources to defer)
   • Resource-Hints-Validation.js (optimize preconnect for slow networks)
3. Cross-reference with webperf-media skill:
   • Image-Element-Audit.js (implement responsive images based on connection)
   • Video-Element-Audit.js (adjust video quality based on connection)

Save-Data Mode Detection

When the user asks about save-data or data-saving features:

1. Network-Bandwidth-Connection-Quality.js - Check if save-data is enabled
2. If save-data is enabled, recommend:
   • Reducing image quality
   • Disabling autoplay videos
   • Deferring non-critical resources
   • Using low-res thumbnails

Mobile/Slow Connection Optimization

When the user asks "optimize for mobile" or "slow connection users":

1. Network-Bandwidth-Connection-Quality.js - Assess connection type
2. Cross-reference with webperf-loading skill:
   • TTFB.js (critical for slow connections)
   • Find-render-blocking-resources.js (minimize blocking on slow networks)
   • Critical-CSS-Detection.js (inline critical CSS to reduce RTT)
   • Prefetch-Resource-Validation.js (avoid excessive prefetch on slow connections)

Decision Tree

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

After Network-Bandwidth-Connection-Quality.js

• If effectiveType is "slow-2g" or "2g" → Very slow connection, recommend:

  1. Run webperf-loading:Critical-CSS-Detection.js (inline critical CSS)
  2. Run webperf-media:Image-Element-Audit.js (implement aggressive lazy loading)
  3. Run webperf-loading:Prefetch-Resource-Validation.js (remove prefetch to save bandwidth)
  4. Run webperf-core-web-vitals:LCP.js (LCP is heavily impacted by slow connections)
  5. Recommend minimal resource strategy

• If effectiveType is "3g" → Moderate connection, recommend:

  1. Run webperf-loading:Find-render-blocking-resources.js (minimize blocking)
  2. Run webperf-media:Image-Element-Audit.js (responsive images)
  3. Run webperf-loading:Resource-Hints-Validation.js (optimize preconnect)
  4. Run webperf-core-web-vitals:INP.js (high latency can impact interaction responsiveness)
  5. Implement adaptive image quality

• If effectiveType is "4g" or better → Good connection, recommend:

  1. Standard optimization practices
  2. Consider strategic prefetch for navigation
  3. Higher quality media is acceptable

• If save-data is enabled → User explicitly wants to save data, recommend:

  1. Reduce image quality aggressively
  2. Disable autoplay videos
  3. Defer non-critical resources
  4. Remove prefetch/preload hints
  5. Show "high quality" toggle option

• If RTT > 300ms → High latency, recommend:

  1. Run webperf-loading:TTFB.js (latency impacts TTFB)
  2. Run webperf-loading:TTFB-Sub-Parts.js (break down latency components)
  3. Run webperf-loading:Resource-Hints-Validation.js (preconnect critical for high RTT)
  4. Run webperf-loading:Service-Worker-Analysis.js (caching is critical for high latency)
  5. Minimize number of origins
  6. Use HTTP/2 or HTTP/3 for multiplexing

• If downlink < 1 Mbps → Very limited bandwidth, recommend:

  1. Run webperf-media:Image-Element-Audit.js (aggressive compression)
  2. Run webperf-media:Video-Element-Audit.js (disable autoplay)
  3. Run webperf-loading:Prefetch-Resource-Validation.js (remove prefetch)
  4. Implement bandwidth-aware loading

• If downlink > 10 Mbps → Good bandwidth, consider:

  • Higher quality media
  • Strategic prefetch
  • Preloading next-page resources

Adaptive Loading Implementation Guide

Based on Network Information API results, implement these strategies:

For slow-2g / 2g (< 50 Kbps): (effectiveType: "slow-2g" or "2g")

• Serve low-res images (quality: 30-40)
• Disable autoplay videos
• Remove all prefetch hints
• Inline all critical CSS
• Defer all non-critical JavaScript
• Use system fonts (no webfonts)
• Aggressive lazy loading (load on scroll + buffer)

For 3g (50-700 Kbps): (effectiveType: "3g")

• Serve medium-res images (quality: 60-70)
• Disable autoplay videos
• Limited prefetch (critical only)
• Inline critical CSS only
• Defer non-critical JavaScript
• Preload 1-2 critical fonts
• Standard lazy loading

For 4g+ (> 700 Kbps): (effectiveType: "4g")

• Serve high-res images (quality: 80-90)
• Allow autoplay videos (muted)
• Strategic prefetch for navigation
• Standard CSS loading and JavaScript loading
• Preload critical fonts
• Standard lazy loading

For save-data mode: (navigator.connection.saveData === true)

• Override connection type, treat as worse than actual
• Show "Load high quality" toggle
• Disable autoplay entirely
• Minimal images, minimal quality
• No prefetch, no preload (except critical)

Performance Budget by Connection Type

Adjust performance budgets based on connection quality:

slow-2g / 2g:

• Total page weight: < 500KB
• Images: < 200KB total
• JavaScript: < 100KB total
• No videos

3g:

• Total page weight: < 1.5MB
• Images: < 800KB total
• JavaScript: < 300KB total
• Videos: < 3MB (only if critical)

4g+:

• Total page weight: < 3MB
• Images: < 2MB total
• JavaScript: < 1MB total
• Videos: < 10MB

Network Information API Limitations

Be aware of API limitations and fallbacks:

API not available:

• Safari does not support Network Information API
• Fallback: Use TTFB as proxy for connection quality
• Fallback: Device detection (mobile = assume slow)
• Fallback: User preference toggle

API values are estimates:

• effectiveType is based on recent observations
• Values can change during session
• Re-check periodically for long sessions

Privacy considerations:

• Some browsers limit precision for privacy
• Values may be rounded or capped
• Consider user privacy when making decisions

References

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