Core Web Vitals: how to prepare ?

  • 126
  • 29 Apr 2021

Did you know?


In May 2021 Google is changing the algorithm of its search engine to take into account the display speed, responsiveness and visual stability of sites


These technical performance indicators are now taken into account by the algorithm as indicators of a good user experience.


This announcement has led to a 70% increase in the number of users of the Google Lighthouse and PageSpeed Insights measurement tools. Don't sit back and watch the competition! Actions can be taken to turn this development into an opportunity.


The new indicators taken into account


Largest Contentful Paint (LCP)


This indicator measures the time it takes to load the page until the largest visible block is displayed on the screen. It should be as short as possible, ideally less than 2.5 seconds.


Core web indicators

On this page, it is only at the last stage that the largest block visible on the screen is loaded  (source:



First Input Delay (FID)


The FID determines the time it takes to react to a user action such as clicking on a button or link. Execution of large or slow javascript code can block the page and degrade this indicator.

First input delay

This site does not respond instantly because the execution of javascript code blocks the page



Cumulative Layout Shift (CLS)


The CLS measures the stability of the site as the page loads. Ideally, each element that is not yet loaded (an image for example) should already have a reserved space when the page is first displayed. If this is not the case, then the appearance of the element will shift the other elements on the page and degrade the CLS.


Cumulative Layout Shift (CLS)

An unstable page while loading (source:


How to turn this change into an opportunity?

Firstly, we can carry out a technical audit of your site; the aim of this audit will be to measure the performance of your site and to identify the technical optimisations to be carried out.


Depending on the characteristics of your platform, we may be able to propose technical optimisations such as:


  • Optimising the loading of javascript / css code
  • Optimising the images and the way they are loaded on the page
  • Modifying the way the site menu is loaded
  • Relying more on the browser cache
  • Pre-loading some critical elements


Contact us, our technical teams can help you turn this algorithm change into an opportunity to boost your website's SEO!



Reference articles

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