Core Web Vitals
Understand LCP, INP and CLS alongside lab diagnostics such as FCP, TBT and Speed Index.
Ludwig Makhyan · Founder, WebsiteCoach ·
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Executive Overview
Core Web Vitals measure loading (LCP), responsiveness (INP) and visual stability (CLS). Lighthouse provides lab diagnostics; CrUX describes eligible real-user experiences. TBT is useful for diagnosing loading-time blocking but is not a Core Web Vital.
Business Impact
Faster and more stable pages can improve the user experience. The effect on conversions depends on the audience and journey; measure it with your own data. Performance scores alone do not establish why a page ranks where it does.
Source: Google Lighthouse lab audit via PageSpeed Insights API. Data is a point-in-time simulation, not real-user data.
Key Metrics Explained
- Largest Contentful Paint (LCP)
- Time until the largest visible content element renders. Identify that element and investigate its loading and rendering delays.
- First Contentful Paint (FCP)
- Time until the first text or image appears. A slow first paint can indicate delayed HTML, fonts, or blocking resources.
- Total Blocking Time (TBT)
- Lab measurement of main-thread blocking during loading. Reduce long tasks and unnecessary JavaScript; TBT is not a Core Web Vital.
- Cumulative Layout Shift (CLS)
- Measures unexpected layout movement. Reserve space for images and embeds and avoid inserting content above what people are reading.
- Speed Index
- Lab estimate of how quickly visible content fills the viewport. Use it to compare similar test conditions, not as a Google ranking score.
- DOM size & render-blocking resources
- Large document trees and blocking resources can increase rendering work. Inspect the actual bottlenecks before removing elements or deferring files.
- Third-party script count & weight
- External scripts add requests and execution work. Review each script's purpose and measure the effect of deferring or removing it.
Business Impact Model
WebsiteCoach's revenue-at-risk figure is an illustrative model using assumed relationships between performance and conversions. It is not a measurement of lost sales:
- LCP:The model assumes roughly 4% conversion loss per second above 2.5 s; validate this assumption for your business.
- TBT:The model uses main-thread blocking as a diagnostic input; it does not measure interaction conversions directly.
- CLS:Layout movement can interrupt tasks. The modeled effect is an assumption, not observed revenue loss.
Use field measurements, conversion analytics and controlled experiments to evaluate actual business impact.
Role-Specific Action Plan
- →Approve sprint allocation for performance work
- →Review CVR impact estimate in the audit report
- →Set Core Web Vitals as quarterly engineering KPIs
- →Enable CDN caching and edge delivery (Cloudflare, Fastly)
- →Optimise TTFB via server caching and compression
- →Ensure HTTP/2 or HTTP/3 is active
- →Preload the LCP image using <link rel='preload'>
- →Defer or remove unused JavaScript bundles
- →Add explicit width and height to all images (prevents CLS)
- →Code-split heavy components to reduce TBT
- →Convert images to WebP or AVIF
- →Audit all installed tracking pixels and tag manager tags
- →Remove unused marketing scripts
- →Consent-gate non-essential trackers to reduce initial load
Frequently asked questions
What is Core Web Vitals?
Understand LCP, INP and CLS alongside lab diagnostics such as FCP, TBT and Speed Index.
Why does Core Web Vitals matter?
Faster and more stable pages can improve the user experience. The effect on conversions depends on the audience and journey; measure it with your own data. Performance scores alone do not establish why a page ranks where it does.
Official references