core-web-vitals

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 on March 12, 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:

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.

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 on March 12, 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