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