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Why you should care about Core Web Vitals and their impact on your eCommerce site

March 30, 2022 / Reading Time: 3 minutes /
Libby Caraher
CRO & UX Research Assistant

It’s no secret that the key to a higher conversion rate is to provide an excellent user experience. A primary way (and Google’s way!), to measure user experience is to assess your website’s Core Web Vitals. These reflect the loading, interactivity, and visual stability of your website.

Put simply, you should care about Core Web Vitals, and deliver across these three areas, to ensure that your website is providing a good user experience to achieve higher conversion rates as well as improve your organic visibility.

core web vitals


A slow loading web page can have a negative impact on the user’s experience of your site, causing increased bounce rates, cart abandonment, and poor opinions of your brand. As a result, it’s really important that you frequently check the speed of your website, as well as key pages visited by users in their user journey. 

To measure the loading speed of a site, Core Web Vitals looks specifically at the Largest Contentful Paint metric, or LCP for short. Largest Contentful Paint is used ‘to determine when the main content of the page has finished rendering on the screen’ The quicker the time in which LCP occurs, the quicker the page content has been rendered- ultimately, providing a better user experience.

So, what should you look to achieve? Google advises that a website with a ‘good user experience’ will have LCP occur ‘within 2.5 seconds of when the page first starts loading’. If you find that your website’s LCP takes longer than this, then you should ensure any images or videos are optimised, check your server response time, and check your website’s code for render blocking JavaScript and CSS.

A quick LCP directly correlates with conversion. An investigation carried out by Renault measured the relationship between a quick LCP load time and conversion rate. Renault found that a lower LCP correlated with lower bounce rates and more form completions (their conversion measurement). This highlights the impact good user experience can have on conversion rate.


Being able to interact with a web page is a crucial component of your website’s usability. In order to provide an excellent user experience, you must ensure your website is actually usable. If your website’s content loads quickly but does not function when the user tries to interact with the page, this creates a negative experience for the user. This could push the user to bounce from the page and affect their decision to return.  

To measure the user’s impression of interactivity, Core Web Vitals uses a metric called First Input Delay, or FID for short. From a technical viewpoint, Philip Walton at Google describes FID measurements as ‘the time from when a user first interacts with a page […] to the time when the browser is actually able to begin processing event handlers in response to that interaction’. In practical terms this is a measurement of the time from when a user presses a button on the page, such as ‘add to cart’,  to the time when the browser begins to process that the button has been clicked, so that the action can be completed – so for our example, that would be the addition of the item to the cart. 

Google considers a low FID measurement of 100 milliseconds of less to provide a good user experience. 

If your website has a high FID measurement, you will need someone with coding knowledge and experience to help reduce this. They will be able to look at reducing the impact of third party code, reducing JavaScript execution time, and minimising main thread work

Visual stability.

To measure visual stability, Core Web Vitals assesses a site’s Cumulative Layout Shift score, or CLS for short. This measures the shifting of elements and buttons, as the user interacts and scrolls a webpage. Moving components of the web page, as the user is interacting or reading the page, causes a jarring user experience, creating feelings of frustration and abandonment of the user journey. 

Google states that a good CLS score is 0.1 or less. If you find that your website is generating CLS scores above 0.1, Google advises ‘size attributes are included on your images and videos’, ‘content is not inserted above existing content (except when reacting to user interaction)’, and ‘use transform animations’ (

Scores below 0.1 indicate that elements on a page do not move as the user interacts with the page. This reflects a positive user experience with a smooth user journey where flow goes uninterrupted to a conversion point. 

Core web vitals

How to test your website’s Core Web Vitals.

To test these three key areas of your website on both mobile and desktop, PageSpeed Insights provides a great overview of your website’s score across both devices. It also provides recommendations tailored specifically to your website on how to improve in each area. Using the results to improve each of these three core areas will improve your overall Core Web Vitals, ultimately leading to a better user experience.

If you are comfortable using the developer tools within Google Chrome, you can also run Core Web Vital checks on individual pages through the ‘Lighthouse’ tool.

Get in touch with us to see how we can help you improve your eCommerce’s website’s Core Web Vitals.

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