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)#

MetricTargetGoodNeeds ImprovementPoorLCP≤2.5s≤2.5s2.5–4.0s>4.0sINP≤200ms≤200ms200–500ms>500msCLS<0.1≤0.10.1–0.25>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:

1. Tools: GSC Core Web Vitals report, PageSpeed Insights, Chrome DevTools
2. Metrics: Which metric is failing (LCP, INP, CLS)
3. 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.

CauseFixSlow server responseReduce TTFB; use CDN; optimize serverRender-blocking resourcesDefer non-critical CSS/JS; inline critical CSSLarge imagesWebP/AVIF; compress; width/ height to prevent CLS; see image-optimization****Client-side renderingSSR/SSG for above-fold content; see rendering-strategies****Third-party scriptsLoad 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.

CauseFixLong-running JSBreak tasks >50ms; use requestIdleCallback; Web WorkersHeavy event handlersDebounce/throttle; defer non-critical workMain thread blockingReduce third-party scripts; defer non-critical JSLayout thrashingBatch DOM reads/writes; avoid forced reflows

CLS Optimization#

CLS measures unexpected layout shifts.

CauseFixImages without dimensionsAlways set width and height attributesDynamic contentReserve space for ads, embeds; use min-heightWeb fontsfont-display: optional or swap; preload critical fontsAnimationsUse 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#

ToolUseGSCCore Web Vitals report; URL grouping; field dataPageSpeed InsightsLab + field data; mobile + desktopChrome DevToolsPerformance 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