Core Web Vitals
Google's set of three performance metrics — LCP, INP, and CLS — that measure page experience and directly influence both search rankings and conversion rates.
Core Web Vitals are a set of three standardised performance metrics defined by Google that measure real-world user experience on webpages. They became official Google ranking signals in 2021 and are also direct conversion rate influencers.
The Three Core Web Vitals
LCP — Largest Contentful Paint
Measures loading performance: the time from page navigation to when the largest visible content element (image, video, or text block) renders in the viewport.
| Score | Rating |
|---|---|
| Under 2.5s | Good ✓ |
| 2.5s – 4.0s | Needs Improvement |
| Over 4.0s | Poor ✗ |
INP — Interaction to Next Paint
Measures responsiveness: the delay between user input (click, tap, keypress) and the browser’s next visual response. Replaced FID (First Input Delay) in March 2024.
| Score | Rating |
|---|---|
| Under 200ms | Good ✓ |
| 200ms – 500ms | Needs Improvement |
| Over 500ms | Poor ✗ |
CLS — Cumulative Layout Shift
Measures visual stability: how much visible content unexpectedly moves during page load. Caused by images without dimensions, dynamic content insertion, and web fonts that cause text reflow.
| Score | Rating |
|---|---|
| Under 0.1 | Good ✓ |
| 0.1 – 0.25 | Needs Improvement |
| Over 0.25 | Poor ✗ |
Core Web Vitals and Conversion Rate
The conversion impact of performance is well-documented:
- 1-second delay in mobile load time → up to 20% conversion drop (Google)
- 0.1-second speed improvement → 8% CVR increase on retail sites (Deloitte)
- High CLS → accidental clicks, frustration, higher exit rate
For CRO, Core Web Vitals are the technical foundation. No amount of headline testing or CTA optimisation compensates for a page that takes 6 seconds to load or shifts its layout as users try to click.
How to Check Your Scores
- PageSpeed Insights: pagespeed.web.dev — enter your URL, get lab and field data
- Google Search Console: Core Web Vitals report — shows real-user data across all pages
- Chrome DevTools: Lighthouse tab — run audits in-browser
Quick Wins for Each Metric
LCP:
- Convert images to WebP format (30–50% smaller than JPEG)
- Add
loading="eager"to above-the-fold images,loading="lazy"to rest - Preload your hero image:
<link rel="preload" as="image" href="hero.webp">
INP:
- Defer non-critical JavaScript
- Remove or delay third-party scripts (chat widgets, analytics tags)
CLS:
- Always set explicit
widthandheighton images and videos - Avoid inserting content above existing content after page load
Core Web Vitals are assessed as part of every technical CRO audit — poor scores are often the hidden reason why otherwise well-designed pages underperform.
Frequently Asked Questions
What are Core Web Vitals?
Core Web Vitals are three Google-defined metrics that measure real-world user experience on a webpage: LCP (Largest Contentful Paint) measures loading speed — how quickly the main content appears; INP (Interaction to Next Paint) measures responsiveness — how fast the page reacts to user input; CLS (Cumulative Layout Shift) measures visual stability — whether elements jump around as the page loads. Google uses these as ranking signals. More importantly for CRO, poor scores directly increase bounce rate and reduce conversions.
How do Core Web Vitals affect conversion rate?
The data is clear: page speed and stability directly impact conversion rate. Google's research shows a 1-second delay in mobile load time can reduce conversions by up to 20%. Deloitte found improving mobile site speed by 0.1 seconds increased conversion rate by 8% for retail sites. CLS issues (content jumping as page loads) cause accidental clicks and frustration, increasing exit rate. Sites with good Core Web Vitals scores consistently outperform poor-scoring pages in both rankings and conversion metrics.
How do you check and improve Core Web Vitals?
Check your scores at PageSpeed Insights (pagespeed.web.dev) or Google Search Console → Core Web Vitals report. For LCP: optimise images (WebP format, lazy loading, proper sizing), use a CDN, and preload critical fonts. For INP: minimise JavaScript execution time and third-party scripts. For CLS: set explicit width/height on all images and videos, avoid dynamically injected content above existing content, and don't use web fonts that cause text reflow. Aim for: LCP under 2.5s, INP under 200ms, CLS under 0.1.