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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…
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.
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