Check Performance

Analyze performance-related HTML patterns on web pages.

Quick Start

cd /path/to/html-checker/scripts
bun src/check-performance.ts <URL>

CLI Options

Option	Short	Default	Description
--verbose	-v	false	Show all resources
--json	-j	false	Output results as JSON

Checks Performed

Check	Impact	Description
Render-blocking CSS	High	CSS in head without media query
Render-blocking JS	High	Scripts without async/defer
No lazy loading	Medium	Images/iframes without loading="lazy"
Missing preconnect	Medium	Third-party domains without preconnect
Missing preload	Medium	Critical resources not preloaded
Large inline styles	Medium	Inline CSS over 14KB
Large inline scripts	Medium	Inline JS over 14KB
No image dimensions	Medium	Images without width/height (CLS)
Unoptimized images	Low	Images without srcset/sizes
No resource hints	Low	Missing dns-prefetch for external domains
Sync third-party	High	Synchronous third-party scripts

Usage Examples

# Basic check
bun src/check-performance.ts https://example.com

# Verbose output
bun src/check-performance.ts https://example.com --verbose

# JSON output
bun src/check-performance.ts https://example.com --json

Output Example

Performance Analysis for https://example.com

Resource Summary:
  Stylesheets: 5 (3 render-blocking)
  Scripts: 12 (4 render-blocking)
  Images: 25 (18 without lazy loading)
  Iframes: 2 (2 without lazy loading)

Critical Issues:
  [HIGH  ] 3 render-blocking CSS files
    - /styles/main.css
    - /styles/vendor.css
    - https://fonts.googleapis.com/css
  [HIGH  ] 4 render-blocking scripts
    - /js/jquery.min.js
    - /js/analytics.js
    - /js/main.js
    - https://third-party.com/widget.js

Optimization Opportunities:
  [MEDIUM] 18 images could use lazy loading
  [MEDIUM] Missing preconnect for 3 domains
    - fonts.googleapis.com
    - third-party.com
    - cdn.example.com
  [MEDIUM] 5 images missing width/height

Resource Hints Found:
  [OK] preconnect: fonts.gstatic.com
  [MISSING] preconnect: third-party.com

Score: 45/100

Recommendations:
  - Add async/defer to non-critical scripts
  - Use media="print" for non-critical CSS
  - Add loading="lazy" to below-fold images
  - Add preconnect for third-party domains
  - Specify width/height on images to prevent CLS

Core Web Vitals Impact

LCP (Largest Contentful Paint)

• Preload critical resources
• Remove render-blocking resources
• Optimize images

CLS (Cumulative Layout Shift)

• Set image dimensions
• Reserve space for ads/embeds
• Avoid injecting content above fold

INP (Interaction to Next Paint)

• Defer non-critical JavaScript
• Break up long tasks
• Use async event handlers

Quick Fixes

<!-- Preconnect to third-party -->
<link rel="preconnect" href="https://fonts.googleapis.com">

<!-- Defer non-critical CSS -->
<link rel="stylesheet" href="print.css" media="print">

<!-- Async/defer scripts -->
<script src="analytics.js" async></script>
<script src="main.js" defer></script>

<!-- Lazy load images -->
<img src="photo.jpg" loading="lazy" width="800" height="600">

Related Files

• Command: plugins/html-checker/commands/check-performance.md
• Scripts: plugins/html-checker/scripts/src/check-performance.ts
• Check Images: plugins/html-checker/skills/check-images/
• Check Meta: plugins/html-checker/skills/check-meta/