If you’ve been doing any reading about SEO in the last few years, you’ve almost certainly come across the term Core Web Vitals. Google has made it clear that page experience matters — and Core Web Vitals are the specific, measurable metrics it uses to assess it. Understanding what they are, how they’re measured, and what you can do about them is now a fundamental part of any serious technical SEO strategy.
What Are Core Web Vitals?
Core Web Vitals are a set of real-world performance metrics that Google uses to measure the quality of a user’s experience on a web page. They focus on three things: how fast the main content loads, how quickly the page responds to user interaction, and how stable the page is visually as it loads. Google introduced Core Web Vitals as an official ranking factor in 2021 as part of its “page experience” signal.
The Three Core Web Vitals Metrics
1. Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visible element on the page to load — typically the hero image, a large header, or a significant block of text. Good: 2.5 seconds or faster. Needs Improvement: 2.5 to 4 seconds. Poor: more than 4 seconds. The most common causes of poor LCP are large unoptimised images, slow server response times, and render-blocking JavaScript and CSS. Our guide to improving page speed for SEO covers the specific fixes in detail.
2. Interaction to Next Paint (INP)
INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. It measures the responsiveness of a page throughout the entire visit — the time between a user interacting with the page and the browser visually responding. Good: 200 milliseconds or less. Needs Improvement: 200–500ms. Poor: over 500ms. Usually caused by heavy JavaScript blocking the browser’s main thread.
3. Cumulative Layout Shift (CLS)
CLS measures visual stability — how much elements on a page move around unexpectedly as it loads. Good: 0.1 or less. Needs Improvement: 0.1–0.25. Poor: over 0.25. CLS is caused by elements loading without reserved space — images without defined dimensions, web fonts that swap after initial render, or dynamically injected content that pushes content down. Fix it by specifying sizes for images and embeds in advance.
How Does Google Measure Core Web Vitals?
Google primarily collects real-world data from actual Chrome users visiting your site — called field data or Chrome User Experience Report (CrUX) data. This means your score in Search Console reflects how real users have actually experienced your pages. A page can score well in Lighthouse but underperform in the field if real users are on slower devices or connections.
Do Core Web Vitals Actually Affect Rankings?
Yes — but not in the way many people imagine. Core Web Vitals are a tiebreaker signal, not a primary ranking factor. A highly relevant page with mediocre Core Web Vitals will still rank above a fast page with poor content. However, in competitive niches where multiple pages have similar quality, Core Web Vitals can tip the balance. The indirect effect is also significant: poor Core Web Vitals cause higher bounce rates, sending negative engagement signals back to Google.
How to Improve Your Core Web Vitals
Improving LCP: Optimise your images — use WebP or AVIF format, compress without quality loss, and serve images at the right display dimensions. Use a CDN. Prioritise your hero image with fetchpriority=”high”. Improve your server response time (TTFB).
Improving INP: Audit your JavaScript for long-running tasks that block the main thread. Defer non-critical scripts and audit third-party scripts (analytics, chat widgets, ad scripts) — these are a common culprit.
Improving CLS: Always define explicit width and height attributes on images and video elements. Use font-display: optional or preload your fonts to reduce layout instability. Avoid inserting dynamic content above existing content unless you reserve space for it.
Prioritising What to Fix
If you have limited developer resources, prioritise anything scoring “Poor” first — these have the most negative impact. Focus on your highest-traffic and most commercially important pages. Look for patterns across the site — template-level changes often fix many pages at once. For the full context on where Core Web Vitals sit within your technical health, a technical SEO audit will help you prioritise effectively.
Continue with the Technical SEO Series
Core Web Vitals sit within the broader discipline of technical SEO. Read The Complete Guide to Technical SEO and explore the other guides on crawl errors, sitemaps, canonicalisation, mobile-first indexing, and structured data.