What are Core Web Vitals? Understanding Google’s crucial performance metrics

By Olga Andrienko, VP Marketing at Semrush on

Google is encouraging all website owners to track and improve three key metrics to deliver better user experiences

Core Web Vitals form a subset of three performance metrics that Google is pushing to improve user experiences across the web. They are known as Largest Contentful Paint (LCP), First Input Delay (FID) and Cumulative Layout Shift (CLS) and, as part of Google’s Page Experience update, are confirmed ranking factors in search.

  • Largest Contentful Paint (LCP): A metric that tracks loading speed of web pages 
  • First Input Delay (FID): A metric that tracks how long it takes for a visitor to interact with a web page
  • Cumulative Layout Shift (CLS): A metric that tracks the stability of visual elements of a web page

One of the greatest signs of the importance of Core Web Vitals is the fact that Google officially announced the part they will play in this summer’s significant update, which put (or should have put) website owners on high alert and firmly in preparation mode.

They aren’t the only search signals for page experience, but they are the ones to keep an eye on insofar as loading, interactivity and visual stability are concerned for the time being.

Why do Core Web Vitals matter?

The driving force behind Core Web Vitals is Google’s desire to create better web experiences for users; in other words, it wants to send people to websites they will enjoy using. If a website returns bad Core Web Vitals, it offers a poor user experience and, so, it will be less likely to rank well in the search engine results pages (SERPs). 

Each metric is concerned with separate aspects of a site’s performance, but they are interconnected to ensure a seamless user experience across the board. If one lets it down—say, for instance, a page’s content takes over 2.5 seconds to load—then the user’s intent is going to be left unfulfilled and they will bounce back to the SERPs to find a better scratch for their itch. 

Whether a webmaster is familiar with Core Web Vitals or not, there isn’t one in the world who would be satisfied with that as an outcome for a single visitor, let alone hundreds or thousands on any given day.

What causes poor scores for Largest Contentful Paint (LCP)?

Let’s face it, load speed (or the lack of it) can be one of the most frustrating things about using some sites on the web. More often than not these days, people are searching and surfing on mobile too, partly because time and convenience are usually of the essence. 

That’s why Google is pushing LCP as one of its core metrics for measuring site performance. It tracks how long it takes for the largest content element of the visible part of the page to appear to the user. Anything under 2.5 seconds is considered ‘Good’, anything between 2.5 and 4 seconds ‘Needs Improvement’ and anything that takes longer than 4 seconds is deemed ‘Poor’.

An unsatisfactory score can be triggered by many things, including slow server response time, JavaScript and CSS that blocks rendering, slow loading of resources like images and video files and client-side rendering.

Anything that takes up a great deal of space will be a red flag when it comes to what is presented to the user, but these issues can be fixed by optimizing the server, adopting a local image CDN to serve global users faster, caching assets and serving HTML pages cache-first and minifying CSS and JavaScript.

What causes poor scores for First Input Delay (FID)?

The overall responsiveness of a web page is measured by tracking the time it takes for a user to interact with its UI, i.e. the time elapsed before a real user event. Tools like Google Lighthouse and Semrush’s Site Audit use a metric called Total Blocking Time (TBT) to measure this, since real user events can’t be replicated.

Think along the lines of long tasks, long JavaScript execution time, large JavaScript bundles and render-blocking JavaScript for an idea of what can have a detrimental effect on FID. A time of under 100ms will secure a ‘Good’ score, while anything that takes longer than 300ms will return a ‘Poor’ rating.

Fixes include reducing execution time of JavaScript, implementing lazy-loading, minimizing unused polyfills and monitoring changes with TBT.

What causes poor scores for Cumulative Layout Shift (CLS)?

The third Core Web Vital is concerned with visual stability. Imagine those infuriating ads, pop-ups and banners that get in the way of page content and can disrupt the layout, and you’ve got a good idea of what triggers poor CLS.

Cookie banners and notices, dynamically injected content, images, embeds and iframes without dimensions and web fonts causing flash of invisible text/flash of unstyled text (FOIT/FOUT) are all in the mix as triggers for bad user experiences, too. A score of 0.1 or under is ‘Good’ and 0.25 or over is ‘Poor’.

The fixes involve including <width> and <height> size attributes on your images and video elements, defining the space for each asset on your page—think ads, banners, responsive images, videos, etc—and loading content downwards when a user lands on the page.

Why you should be monitoring and optimizing Core Web Vitals

There are many benefits to monitoring and optimizing Core Web Vitals as a website owner, from protecting and improving rankings to elevating user experiences. Frankly, the reality of avoiding or ignoring it is quite simply not worth the risk.

Even prior to the rollout of Google’s update, which is expected to be complete for all URLs by the end of August, a Semrush study of Core Web Vitals found that pages with ‘Good’ vital ratings were already performing well in search. Alarmingly, it also flagged that there’s still work to be done on many mobile sites, despite Google banging that drum for many years now, as only 16.2% of mobile URLs achieved a ‘Good’ score on all three Core Web Vitals.

Those sites that aren’t ready for the update risk not only losing rankings, but also customers as a result. Indeed, underestimating the power of search and the delivery of a great user experience is a costly game to play.

Find out more about Semrush on their CMA member page.

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