Core Web Vitals explained: LCP, INP and CLS, and how to improve page speed

Short answer: Core Web Vitals are three measures Google uses to judge how real visitors experience a web page. LCP measures how quickly the largest content loads, INP how quickly the page responds when someone clicks, and CLS how much the content jumps around. Good scores are 2.5 seconds, 200 milliseconds and 0.1. They are used in Google's rankings, but weigh less than how relevant the content is.

The three measures

The thresholds come from Google, which recommends that 75 percent of visits should meet them, measured separately for mobile and desktop (web.dev).

• LCP is the time until the largest visible element, usually an image, video or block of text at the top, has been shown (web.dev).
• INP follows every click, tap and key press during the visit and reports roughly the slowest. It replaced FID, which only measured the first click, on 12 March 2024 (web.dev, web.dev).
• CLS measures the largest burst of unexpected layout shifts, for example when an ad loads and pushes down the text you were reading (web.dev).

Do Core Web Vitals affect rankings?

Yes, but not most. Google says Core Web Vitals are used by its ranking systems, but that there is no single signal and relevance still matters most. When several pages are equally good answers, a better page experience can decide it (Google Search Central). Google still strongly recommends that site owners achieve good Core Web Vitals (Google Search Central).

The biggest gain is often not the ranking but the visitors: fast, stable pages mean more people stay and take the next step. Read more about how ranking works in How does SEO work?

How to test page speed

There are two kinds of data. Field data comes from real Chrome users and is what Google uses. Lab data comes from a test in a simulated environment and is good for finding causes. A lab test cannot measure INP properly and uses Total Blocking Time as a stand-in (web.dev).

• PageSpeed Insights shows field data from the last 28 days and a Lighthouse lab test with a score from 0 to 100 (Google). Do not obsess over the score: a page scoring 70 can still pass Core Web Vitals for real users.
• Google Search Console has a Core Web Vitals report that groups your pages by experience and shows which are good, need improvement or are poor, separately for mobile and desktop (Google Search Console Help). Read Google indexing for how to set up Search Console.

If your site has too little traffic, there is no field data, and you have to rely on the lab test.

How to improve LCP

• Compress and resize the large image at the top, and use modern formats such as WebP or AVIF.
• Make sure the image is in the page's HTML and give it high priority with fetchpriority="high".
• Never lazy-load the image at the top. It should load first.
• Reduce render-blocking CSS and JavaScript, and defer scripts that are not needed straight away.
• Improve server response time with caching and fewer redirects. Hosting close to your visitors, for UK customers in the UK or nearby, often helps.

Google splits LCP into four parts: server response time, the delay before the image starts loading, the load time and the time before it is shown, which makes it easier to see where the time goes (web.dev).

How to improve INP

• Break up long JavaScript tasks so the browser has time to respond to clicks.
• Remove scripts you do not need, often old tracking codes, chat widgets and plugins.
• Keep the page's HTML small. A large DOM makes every update heavier.
• Show straight away that a click has registered, even if the result takes a moment.

Long tasks that block the main thread are the most common cause of poor INP (web.dev).

How to improve CLS

• Set width and height on all images and videos.
• Reserve space for ads, embeds and cookie banners.
• Load fonts so the text does not jump when the font swaps.
• Never insert new content above what the visitor is already reading.

What it means for business

Most sites have a lot to gain. According to HTTP Archive, only 48 percent of sites passed all three measures on mobile in 2025, and 56 percent on desktop (HTTP Archive Web Almanac). Case studies published by Google:

• Vodafone improved LCP by 31 percent in an A/B test and got 8 percent more sales (web.dev).
• redBus improved INP by 72 percent and increased sales by 7 percent (web.dev).
• The Economic Times cut INP from over 1,000 to 257 milliseconds, halved its bounce rate and got 43 percent more page views on the affected pages (web.dev).

A faster site also lifts conversion. Read What is a good conversion rate?