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:

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 <head>, 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 <img> and <video> elements, use CSS aspect-ratio

Do ads or embeds inject content?

→ Dynamic content pushing existing content down
Fix: reserve space for ad slots with min-height, use contain-intrinsic-size for lazy content

Do fonts cause text reflow?

→ FOUT (Flash of Unstyled Text) causes layout shift when custom font loads
Fix: use font-display: optional (no swap = no shift), or match fallback font metrics

Does dynamic content insert above the fold?

→ Banners, cookie notices, notifications pushing content
Fix: use overlays instead of inline insertions, or reserve space with fixed-height containers

INP Root-Cause Tree

INP measures responsiveness to user interactions. Diagnose:

Is the main thread blocked by long tasks?

Check: JavaScript execution time, third-party scripts, large DOM
Fix: break long tasks with requestIdleCallback or setTimeout, code-split heavy modules

Do event handlers do heavy synchronous work?

Check: click handlers that trigger large DOM updates, form validation on every keystroke
Fix: debounce inputs, use requestAnimationFrame for visual updates, offload work to web workers

Are third-party scripts competing for the main thread?

Check: analytics, chat widgets, A/B testing tools, social embeds
Fix: defer loading until after interaction, use loading="lazy" for embeds, consider removing low-value scripts

Resource Analysis

Break down the total page weight:

| Resource Type | Size | Assessment | Action | |---|---|---|---| | HTML | [x] KB | [ok/large] | Compress, reduce inline styles/scripts | | CSS | [x] KB | [ok/large] | Remove unused CSS, minify, critical CSS extraction | | JavaScript | [x] KB | [ok/large] | Code-split, tree-shake, defer non-critical | | Images | [x] KB | [ok/large] | Modern formats, responsive sizes, lazy load below fold | | Fonts | [x] KB | [ok/large] | Subset, limit families/weights, preload critical | | Third-party | [x] KB | [ok/large] | Audit necessity, defer, self-host if possible |

Benchmarks:

Total page weight under 1.5 MB is good
JavaScript under 300 KB (compressed) for most sites
CSS under 100 KB (compressed)
First-party fonts under 100 KB

Output Format

Speed Audit: [URL or domain]

Core Web Vitals

| Metric | Value | Rating | Root Cause | |--------|-------|--------|------------| | LCP | [value] | Good / Needs Improvement / Poor | [identified cause] | | CLS | [value] | ... | ... | | INP | [value] | ... | ... | | FCP | [value] | ... | ... | | TTFB | [value] | ... | ... |

Resource Breakdown [Table from Resource Analysis]

Priority Fixes

For each failing metric, ordered by impact:

[Metric]: [Root cause]
- Current: [value]
- Target: [threshold]
- Fix: [specific action]
- Estimated impact: [high/medium/low]
...

Quick Wins List optimizations that require minimal effort:

[ ] Add width/height to images
[ ] Set fetchpriority="high" on LCP image
[ ] Defer non-critical JavaScript
[ ] ...

Pro Tip: Use the free CWV Impact Calculator to estimate the traffic impact of fixing Core Web Vitals, and the Critical CSS Generator to extract above-the-fold CSS. SEOJuice MCP users can run /seojuice:page-audit [domain] [url] for instant CWV scores, Lighthouse data, and resource breakdowns.

SkillForge

audit-speed

Before / After 效果对比

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