Quick answer
Core Web Vitals are three measures of real-world page experience: Largest Contentful Paint (loading, good at 2.5 seconds or less), Interaction to Next Paint (responsiveness, 200 milliseconds or less) and Cumulative Layout Shift (visual stability, 0.1 or less), assessed at the 75th percentile of visits.
The three metrics
Google’s Core Web Vitals focus on loading, interactivity and stability. The published “good” thresholds are below, measured for the 75th percentile of page loads, separately for mobile and desktop.
- Largest Contentful Paint (LCP): how quickly the main content appears. Good is 2.5 seconds or less.
- Interaction to Next Paint (INP): how quickly the page responds to taps and clicks. Good is 200 milliseconds or less.
- Cumulative Layout Shift (CLS): how much content jumps around while loading. Good is 0.1 or less.
Do they affect rankings?
Page experience is one of many signals Google uses, and great content still matters most. But slow, janky pages lose visitors and conversions regardless of rankings, so passing Core Web Vitals is worth doing for users first. Google’s own guidance is in its Search Central documentation.
How to measure them
Use field data (real users) as the source of truth and lab tools to debug.
- Google Search Console: the Core Web Vitals report groups URLs by real-user performance
- PageSpeed Insights: shows field data when available, plus lab diagnostics
- Chrome DevTools and Lighthouse: reproduce and debug problems locally
- Real-user monitoring: track performance continuously on your own site
Fixes that move the needle
Most gains come from a few well-known changes.
- LCP: optimise and properly size the hero image, preload key assets, use a fast host or CDN, remove render-blocking scripts
- INP: reduce heavy JavaScript, break up long tasks, avoid unnecessary re-rendering
- CLS: reserve space for images, ads and embeds with width and height, and load fonts without layout jumps
Common causes, by symptom
- Slow LCP: oversized hero images, slow servers, render-blocking CSS or scripts
- Poor INP: heavy JavaScript, long tasks, third-party tags
- High CLS: images without dimensions, late-loading ads, web-font swaps
A 30-minute performance audit
- Run PageSpeed Insights on your key templates
- Check the Search Console Core Web Vitals report
- List the three largest images and scripts
- Remove or defer third-party tags you do not need
- Set a performance budget and re-test after each release
Build it in, then keep it healthy
The cheapest way to pass is to design for performance from the start, with budgets for page weight and script cost. Our web development service does this by default. After launch, performance drifts as plugins and content pile up, which is why ongoing checks are part of our website maintenance work and our maintenance plans. Core Web Vitals also sit within wider technical SEO.
If you are deciding between design and build work, see web design vs web development.
Hosting and delivery choices affect both speed and spend, so read cloud cost optimisation alongside this guide, and keep your site healthy over time with security and patching basics.
Frequently asked questions
What replaced First Input Delay?
Interaction to Next Paint (INP) replaced First Input Delay as the responsiveness metric in Core Web Vitals.
Do I need a perfect score?
No. Aim for “good” on all three at the 75th percentile, then focus on real user experience.
Why do my lab and field results differ?
Lab tests simulate one device and connection. Field data reflects real users on many devices, so it is the more reliable measure.
Sources and further reading
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