SEO Technical: Core Web Vitals

Guides optimization of Core Web Vitals (CWV)—Google's user experience metrics that affect search ranking. CWV are confirmed ranking factors for mobile and desktop.

When invoking: On first use, if helpful, open with 1–2 sentences on what this skill covers and why it matters, then provide the main output. On subsequent use or when the user asks to skip, go directly to the main output.

Scope (Technical SEO)

• LCP (Largest Contentful Paint): Loading performance; time to render largest content element

• INP (Interaction to Next Paint): Responsiveness; replaced FID in March 2024

• CLS (Cumulative Layout Shift): Visual stability; unexpected layout shifts

Target Thresholds (75th percentile, field data)

Metric Target Good Needs Improvement Poor

LCP ≤2.5s ≤2.5s 2.5–4.0s

4.0s

INP ≤200ms ≤200ms 200–500ms

500ms

CLS <0.1 ≤0.1 0.1–0.25

0.25

Source: Google Page Experience

Initial Assessment

Check for project context first: If .claude/project-context.md or .cursor/project-context.md exists, read it for site URL.

Identify:

• Tools: GSC Core Web Vitals report, PageSpeed Insights, Chrome DevTools

• Metrics: Which metric is failing (LCP, INP, CLS)

• Page type: Hero, article, product, list—LCP candidate differs

LCP Optimization

LCP measures the time until the largest content element (image, video, or text block) is visible.

Cause Fix

Slow server response Reduce TTFB; use CDN; optimize server

Render-blocking resources Defer non-critical CSS/JS; inline critical CSS

Large images WebP/AVIF; compress; width /height to prevent CLS; see image-optimization

Client-side rendering SSR/SSG for above-fold content; see rendering-strategies

Third-party scripts Load async; defer non-critical

LCP candidates: Hero image, large text block, video poster. Ensure above-fold images use loading="eager" (default); never lazy-load LCP.

INP Optimization

INP measures responsiveness—time from user interaction to next paint. Replaced FID in March 2024.

Cause Fix

Long-running JS Break tasks >50ms; use requestIdleCallback ; Web Workers

Heavy event handlers Debounce/throttle; defer non-critical work

Main thread blocking Reduce third-party scripts; defer non-critical JS

Layout thrashing Batch DOM reads/writes; avoid forced reflows

CLS Optimization

CLS measures unexpected layout shifts.

Cause Fix

Images without dimensions Always set width and height attributes

Dynamic content Reserve space for ads, embeds; use min-height

Web fonts font-display: optional or swap ; preload critical fonts

Animations Use transform instead of top /left /width

Reserve space: For images, ads, embeds—define dimensions before load. Avoid inserting content above existing content without reserved space.

Tools & Monitoring

Tool Use

GSC Core Web Vitals report; URL grouping; field data

PageSpeed Insights Lab + field data; mobile + desktop

Chrome DevTools Performance panel; LCP element; layout shift overlay

Output Format

• Current state: Which metrics fail (LCP, INP, CLS)

• Prioritized fixes: By impact

• References: Web Vitals, Page Experience

Related Skills

• image-optimization: LCP image optimization; WebP; lazy loading (below-fold only)

• google-search-console: CWV report; field data monitoring

• mobile-friendly: Mobile-first indexing; mobile CWV targets

• rendering-strategies: SSR/SSG for LCP; content in initial HTML

• site-crawlability: Redirect chains waste crawl; fix for performance