WebPerf: Loading Performance

JavaScript snippets for measuring web performance in Chrome DevTools. Execute with mcp__chrome-devtools__evaluate_script, capture output with mcp__chrome-devtools__get_console_message.

Scripts

• scripts/Back-Forward-Cache.js — Back/Forward Cache (bfcache)
• scripts/CSS-Media-Queries-Analysis.js — CSS Media Queries Analysis
• scripts/Client-Side-Redirect-Detection.js — Client-Side Redirect Detection
• scripts/Content-Visibility.js — Content Visibility
• scripts/Critical-CSS-Detection.js — Critical CSS Detection
• scripts/Event-Processing-Time.js — Event Processing Time
• scripts/FCP.js — First Contentful Paint (FCP)
• scripts/Find-Above-The-Fold-Lazy-Loaded-Images.js — Find Above The Fold Lazy Loaded Images
• scripts/Find-Images-With-Lazy-and-Fetchpriority.js — Find Images With Loading Lazy and Fetchpriority
• scripts/Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js — Find non Lazy Loaded Images outside of the viewport
• scripts/Find-render-blocking-resources.js — Find render-blocking resources
• scripts/First-And-Third-Party-Script-Info.js — First And Third Party Script Info
• scripts/First-And-Third-Party-Script-Timings.js — First And Third Party Script Timings
• scripts/Fonts-Preloaded-Loaded-and-used-above-the-fold.js — Fonts Preloaded, Loaded, and Used Above The Fold
• scripts/Inline-CSS-Info-and-Size.js — Inline CSS Info and Size
• scripts/Inline-Script-Info-and-Size.js — Inline Script Info and Size
• scripts/JS-Execution-Time-Breakdown.js — JavaScript Execution Time Breakdown
• scripts/Prefetch-Resource-Validation.js — Prefetch Resource Validation
• scripts/Priority-Hints-Audit.js — Priority Hints Audit
• scripts/Resource-Hints-Validation.js — Resource Hints Validation
• scripts/Resource-Hints.js — Resource Hints
• scripts/SSR-Hydration-Data-Analysis.js — SSR Framework Hydration Data Analysis
• scripts/Script-Loading.js — Scripts Loading
• scripts/Service-Worker-Analysis.js — Service Worker Analysis
• scripts/TTFB-Resources.js — Time To First Byte: Measure TTFB for all resources
• scripts/TTFB-Sub-Parts.js — Time To First Byte: Measure TTFB sub-parts
• scripts/TTFB.js — Time To First Byte: Measure the time to first byte
• scripts/Validate-Preload-Async-Defer-Scripts.js — Validate Preload on Async/Defer Scripts

Descriptions and thresholds: references/snippets.md

Common Workflows

Complete Loading Performance Audit

When the user asks for a comprehensive loading analysis or "audit loading performance":

1. TTFB.js - Establish baseline server/network performance
2. FCP.js - Check initial render timing
3. Find-render-blocking-resources.js - Identify what's blocking rendering
4. Critical-CSS-Detection.js - Validate CSS strategy
5. Script-Loading.js - Audit script loading patterns
6. Resource-Hints-Validation.js - Check optimization hints

Server/Backend Performance Investigation

When TTFB is slow or the user asks "why is my server slow":

1. TTFB.js - Measure overall TTFB
2. TTFB-Sub-Parts.js - Break down into DNS, connection, server time
3. Service-Worker-Analysis.js - Check for SW overhead impacting TTFB
4. TTFB-Resources.js - Identify slow third-party or API endpoints

Font Loading Optimization

When the user asks about fonts, FOIT, FOUT, or font performance:

1. Fonts-Preloaded-Loaded-and-used-above-the-fold.js - Full font audit
2. Resource-Hints-Validation.js - Verify font preloads are correct
3. Find-render-blocking-resources.js - Check if fonts block rendering

Script Performance Deep Dive

When scripts are suspected to slow down the page:

1. Script-Loading.js - Identify blocking scripts and loading strategy
2. First-And-Third-Party-Script-Info.js - Separate first vs third-party impact
3. First-And-Third-Party-Script-Timings.js - Diagnose slow script connections
4. JS-Execution-Time-Breakdown.js - Network vs parse/execution time
5. Inline-Script-Info-and-Size.js - Measure inline script overhead
6. Validate-Preload-Async-Defer-Scripts.js - Find preload anti-patterns

Resource Hints & Priority Optimization

When the user wants to optimize resource loading priorities:

1. Resource-Hints.js - Overview of all hints in use
2. Resource-Hints-Validation.js - Verify hints are actually used
3. Priority-Hints-Audit.js - Check fetchpriority usage
4. Prefetch-Resource-Validation.js - Validate prefetch strategy
5. Validate-Preload-Async-Defer-Scripts.js - Find conflicts

CSS Optimization Workflow

When CSS is bloated or blocking rendering:

1. Critical-CSS-Detection.js - Check critical CSS strategy
2. Inline-CSS-Info-and-Size.js - Measure inline CSS overhead
3. CSS-Media-Queries-Analysis.js - Find unused responsive CSS
4. Find-render-blocking-resources.js - Identify blocking stylesheets

Image Loading Audit

When images are suspected to cause loading issues:

1. Find-Above-The-Fold-Lazy-Loaded-Images.js - Check for lazy-loading anti-patterns
2. Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js - Find missed optimization opportunities
3. Find-Images-With-Lazy-and-Fetchpriority.js - Detect contradictory attributes
4. Priority-Hints-Audit.js - Verify LCP image has fetchpriority="high"

SSR/Framework Performance

When analyzing Next.js, Nuxt, Remix, or other SSR frameworks:

1. SSR-Hydration-Data-Analysis.js - Analyze hydration data size
2. Script-Loading.js - Check framework script loading patterns
3. JS-Execution-Time-Breakdown.js - Measure hydration execution cost
4. Content-Visibility.js - Check if content-visibility is used for optimization

Decision Tree

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

After TTFB.js

• If TTFB > 600ms → Run TTFB-Sub-Parts.js to diagnose where time is spent
• If Service Worker detected → Run Service-Worker-Analysis.js to check for SW overhead
• If TTFB varies significantly across resources → Run TTFB-Resources.js

After FCP.js

• If FCP > 1.8s → Run:
  1. Find-render-blocking-resources.js (CSS/JS blocking)
  2. Critical-CSS-Detection.js (CSS strategy)
  3. Script-Loading.js (blocking scripts)
• If FCP is good but user complains about perceived slowness → Check LCP with webperf-core-web-vitals skill

After Find-render-blocking-resources.js

• If blocking stylesheets found → Run Critical-CSS-Detection.js
• If blocking scripts found → Run:
  1. Script-Loading.js (loading strategy)
  2. Validate-Preload-Async-Defer-Scripts.js (check for anti-patterns)
• If fonts blocking → Run Fonts-Preloaded-Loaded-and-used-above-the-fold.js

After Script-Loading.js

• If many blocking/parser-blocking scripts → Run:
  1. JS-Execution-Time-Breakdown.js (measure execution cost)
  2. First-And-Third-Party-Script-Info.js (identify third-party culprits)
• If third-party scripts detected → Run First-And-Third-Party-Script-Timings.js
• If large inline scripts → Run Inline-Script-Info-and-Size.js

After Resource-Hints-Validation.js

• If unused preloads found → Recommend removing them
• If missing preloads for critical resources → Run:
  1. Fonts-Preloaded-Loaded-and-used-above-the-fold.js (fonts)
  2. Priority-Hints-Audit.js (LCP candidate)
• If preloads on async/defer scripts → Run Validate-Preload-Async-Defer-Scripts.js

After Service-Worker-Analysis.js

• If SW overhead > 100ms → Recommend Navigation Preload API
• If SW caching many resources → Run TTFB-Resources.js to verify cache hits
• If SW not detected but site is SPA/PWA → Recommend implementing SW

After Fonts-Preloaded-Loaded-and-used-above-the-fold.js

• If fonts preloaded but not used above-the-fold → Recommend removing preloads
• If fonts used but not preloaded → Recommend adding preload
• If many font variants loaded → Suggest subsetting or reducing variants

After First-And-Third-Party-Script-Info.js

• If many third-party scripts (>5) → Run:
  1. First-And-Third-Party-Script-Timings.js (detailed timing)
  2. JS-Execution-Time-Breakdown.js (execution impact)
• If third-party scripts are slow → Recommend async/defer or removal

After SSR-Hydration-Data-Analysis.js

• If hydration data > 100KB → Recommend optimization strategies
• If multiple hydration scripts → Investigate framework configuration
• If large inline hydration data → Consider streaming or chunking

After Priority-Hints-Audit.js

• If LCP candidate lacks fetchpriority="high" → Recommend adding it
• If conflicting priorities (preload + low) → Recommend resolving conflict
• If fetchpriority on non-critical resources → Review priority strategy

After Prefetch-Resource-Validation.js

• If >10 prefetch hints → Recommend reducing to critical resources
• If individual prefetch > 500KB → Question necessity
• If total prefetch > 2MB → Flag as mobile bandwidth concern

After Critical-CSS-Detection.js

• If render-blocking CSS > 14KB → Recommend inlining critical CSS
• If no inline CSS but has blocking stylesheets → Suggest critical CSS extraction
• If CSS > 50KB total → Run CSS-Media-Queries-Analysis.js to find savings

After Back-Forward-Cache.js

• If bfcache blocked → Provide specific remediation based on blocking reasons
• If bfcache eligible → Confirm optimization is working

After Client-Side-Redirect-Detection.js

• If client-side redirects detected → Recommend server-side redirects
• If redirect chain found → Suggest eliminating intermediate hops

References

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

Execute via mcp__chrome-devtools__evaluate_script → read with mcp__chrome-devtools__get_console_message.