---
name: audit-speed
description: >
Deep Core Web Vitals and page speed audit. Use when the user asks about page
speed, Core Web Vitals, LCP, CLS, INP, FCP, TTFB, Lighthouse scores, why a
page is slow, performance optimization, or resource size analysis. For broader
technical SEO issues, see diagnose-seo.
metadata:
version: 1.0.0
---
# Audit Speed
Deep Core Web Vitals audit with root-cause analysis trees and resource
optimization recommendations.
## Core Web Vitals Thresholds
| Metric | Good | Needs Improvement | Poor |
|--------|------|-------------------|------|
| LCP (Largest Contentful Paint) | < 2.5s | 2.5s - 4.0s | > 4.0s |
| CLS (Cumulative Layout Shift) | < 0.1 | 0.1 - 0.25 | > 0.25 |
| INP (Interaction to Next Paint) | < 200ms | 200ms - 500ms | > 500ms |
| FCP (First Contentful Paint) | < 1.8s | 1.8s - 3.0s | > 3.0s |
| TTFB (Time to First Byte) | < 800ms | 800ms - 1800ms | > 1800ms |
## Before You Start
Gather this context:
1. **Which pages?** Homepage, key landing pages, or specific slow pages.
2. **Current scores.** If the user has Lighthouse or PageSpeed Insights data, start there.
3. **Tech stack.** CMS, framework, hosting — this determines which optimizations are available.
4. **Known constraints.** Third-party scripts they can't remove, design requirements that limit optimization.
If no data is available, suggest running Google PageSpeed Insights on the key URLs.
## LCP Root-Cause Tree
LCP measures when the largest visible element finishes rendering. Diagnose:
**Is TTFB slow (> 800ms)?**
- → Server response time issue
- Check: hosting quality, CDN configuration, database queries, server-side rendering time
- Fix: upgrade hosting, add CDN, optimize server-side code, enable caching
**Is the LCP element an image?**
- → Image optimization issue
- Check: image format (use WebP/AVIF), image size (serve responsive sizes), lazy loading on LCP image (should NOT be lazy loaded)
- Fix: convert to modern formats, add `width`/`height` attributes, use `fetchpriority="high"` on LCP image, preload the LCP image
**Is the LCP element text?**
- → Font loading issue
- Check: custom fonts blocking render, font file size, font-display strategy
- Fix: use `font-display: swap` or `optional`, preload critical fonts, subset fonts to used characters
**Is render-blocking CSS/JS delaying the LCP?**
- Check: large CSS files in ``, synchronous JS before content
- Fix: inline critical CSS, defer non-critical CSS, async/defer JS
## CLS Root-Cause Tree
CLS measures unexpected layout shifts. Diagnose:
**Do images/videos lack dimensions?**
- → Browser can't reserve space before loading
- Fix: add `width` and `height` attributes to all `![]()
` and `