Complete Guide to Core Web Vitals in 2025

What Are Core Web Vitals?

Core Web Vitals are a set of specific factors that Google considers important for a webpage's overall user experience. They are a subset of Web Vitals, an initiative by Google to provide unified guidance for quality signals that are essential for delivering a great user experience on the web.

The Three Core Web Vitals

1. Largest Contentful Paint (LCP)

What it measures: Loading performance - specifically, the time it takes for the largest content element to become visible.

Target: LCP should occur within 2.5 seconds of when the page first starts loading.

Common causes of poor LCP:

Slow server response times

Render-blocking JavaScript and CSS

Slow resource load times

Client-side rendering

How to improve:

Optimize your server

Use a CDN

Preload important resources

Optimize and compress images

2. Interaction to Next Paint (INP)

What it measures: Responsiveness - how quickly your page responds to user interactions throughout the entire page lifecycle.

Target: INP should be 200 milliseconds or less.

Note: INP replaced First Input Delay (FID) as a Core Web Vital in March 2024.

Common causes of poor INP:

Long JavaScript tasks

Large DOM size

Heavy third-party scripts

Inefficient event handlers

How to improve:

Break up long tasks

Optimize event callbacks

Reduce DOM size

Minimize third-party code

3. Cumulative Layout Shift (CLS)

What it measures: Visual stability - how much the page layout shifts during loading.

Target: Pages should maintain a CLS of 0.1 or less.

Common causes of poor CLS:

Images without dimensions

Ads, embeds, and iframes without dimensions

Dynamically injected content

Web fonts causing FOIT/FOUT

How to improve:

Always include size attributes on images and videos

Reserve space for ad slots

Avoid inserting content above existing content

Preload fonts

How Core Web Vitals Affect SEO

Since June 2021, Core Web Vitals have been part of Google's ranking signals. While content relevance remains the most important factor, good Core Web Vitals can give you an edge over competitors with similar content quality.

The Page Experience Update

Google's page experience signals include:

Core Web Vitals (LCP, INP, CLS)

Mobile-friendliness

HTTPS security

No intrusive interstitials

Measuring Core Web Vitals

Lab Data vs. Field Data

Lab Data: Collected in a controlled environment

Lighthouse

Chrome DevTools

WebPageTest

Field Data: Collected from real users

Chrome User Experience Report (CrUX)

PageSpeed Insights

Search Console

Tools for Measurement

PageSpeed Insights: Shows both lab and field data

Chrome DevTools: Real-time performance monitoring

Search Console: Core Web Vitals report for your entire site

BulkAudit: Audit multiple pages simultaneously

Best Practices for 2025

Prioritize INP: With the switch from FID, focus on overall interactivity

Monitor continuously: Use Real User Monitoring (RUM)

Test on real devices: Don't rely solely on lab data

Optimize for mobile first: Most users are on mobile devices

Regular audits: Use tools like BulkAudit to check multiple pages

Conclusion

Core Web Vitals are essential metrics for understanding and improving your website's user experience. By focusing on LCP, INP, and CLS, you can create faster, more responsive, and more stable web pages that both users and search engines will love.