What are Core Web Vitals?
Google groups metrics that show how visitors experience a page under the name Web Vitals. According to web.dev, the three most important of these are the Core Web Vitals:
- Largest Contentful Paint (LCP) measures loading.
- Interaction to Next Paint (INP) measures responsiveness.
- Cumulative Layout Shift (CLS) measures visual stability.
The selection can change. In March 2024, INP replaced the previous metric, First Input Delay (FID). FID only measured the delay of the first input, whereas INP considers all clicks, taps and key presses during a visit.
Core Web Vitals are one part of user experience. They do not measure whether a page is well designed or easy to understand, only whether it loads quickly, responds quickly and stays still.
LCP, INP and CLS: what is measured and which values apply
- LCP: the time until the largest visible element in the first screen is displayed. This is usually a large image, the poster image of a video or a block of text. Good is 2.5 seconds or less, poor is more than 4 seconds.
- INP: the time until the page visibly responds to a click, tap or key press. Scrolling and hovering do not count. Good is 200 milliseconds or less, poor is more than 500 milliseconds.
- CLS: how much content shifts unexpectedly, for example when text jumps down because an image loads above it. Shifts that occur within 500 milliseconds of a click do not count. Good is a value of 0.1 or less, poor is a value above 0.25.
Values between “good” and “poor” are classed as needing improvement. What counts is the 75th percentile of page loads, segmented by mobile and desktop. A page therefore only meets the targets when each of the three values is in the good range for at least 75 per cent of page loads. A few fast measurements on an office PC are not enough.
What role do Core Web Vitals play in rankings?
In its documentation on page experience, Google states that Core Web Vitals are used by its ranking systems. At the same time, Google makes clear that good values do not guarantee a place at the very top. Search shows the most relevant content, even if its page experience is weak.
Core Web Vitals are therefore one of many ranking factors and do not replace good content. They matter where many pages offer similarly helpful answers. For such queries, Google says a good page experience can contribute to success. Regardless of rankings, a page that loads slowly or jumps while you read annoys the visitors who are already there.
Measuring Core Web Vitals: field data and lab data
There are two kinds of measurement that are often confused:
- Field data comes from real visitors using Chrome. Google collects it in the Chrome User Experience Report over a rolling 28-day window. The Core Web Vitals report in Google Search Console is based on this data. Search Console groups similar URLs together, so that pages with few visitors also receive a value.
- Lab data comes from a simulated measurement, for example with Lighthouse in Chrome’s developer tools. It is useful for finding causes and testing changes immediately. INP cannot be measured directly in the lab because it needs real input. Lighthouse shows Total Blocking Time as an indicator instead.
PageSpeed Insights shows both side by side. If they differ, go by the field data: it shows what real visitors experience, and the Search Console report is based on it. Because it is collected over 28 days, you will only see an improvement there after a few weeks.
Typical causes of poor values
LCP suffers mainly from:
- a slow first response from the server,
- large, uncompressed images in the first screen,
- a main image that is only loaded via JavaScript or with deferred loading (lazy loading).
INP gets worse when long JavaScript tasks block the browser. A frequent source is a large number of third-party scripts, such as tracking and marketing tags loaded via Google Tag Manager.
CLS is usually caused by images and videos without dimensions, by ad slots and embedded elements that change their size afterwards, and by web fonts that render larger or smaller than the fallback font shown first.
Which cause applies to your website only becomes clear when you measure each page type. The home page, category pages and product pages often have quite different problems. This analysis is part of technical SEO.
Are your Core Web Vitals in the red and you are not sure where to start? Then get in touch with us or read more about our technical SEO!
