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:
Which pages?
Homepage, key landing pages, or specific slow pages.
Current scores.
If the user has Lighthouse or PageSpeed Insights data, start there.
Tech stack.
CMS, framework, hosting — this determines which optimizations are available.
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
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