The Bible of Core Web Vitals

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In 2020, Google announced the Core Web Vitals, a series of factors that, according to Google, would begin to be counted within the Google positioning algorithm starting in May 2021, thus affecting the positioning of websites.

In the following article, we will learn what Core Web Vitals are, how they affect positioning, how to improve them and how to manage them with the most important CMS on the market.

What are Core Web Vitals?
In summary, we could say that there are three metrics focused on user experience. These metrics are aimed at measuring the loading speed of the website, interactivity and what would be the visual stability of the website.

These metrics, along with other factors such as pop-up removal, security, and mobile optimization, form the signals that Google uses to evaluate user experience.


The goal of these three metrics is for Google to detect what the first impression the user will have when viewing the page. According to Google, websites that meet these benchmarks focused on obtaining a better impression for the user are 24% less likely to lose users during the web page loading process.

Below we will explain the characteristics of the metrics that make up the Core Web Vitals:

Largest Contentful Paint (LCP)
First Input Delay (FID)
Cumulative Layout Shift (CLS)
Largest Contentful Paint
The Largest Contentful Paint measures loading performance, and we're not talking about measuring page speed, but rather the perception of page speed. In other words, what LCP is measuring is when the user thinks the interior designers service email database page is loaded. Therefore, it's measuring how quickly users see the content.


Factors influencing this event:

Server response
Loading the different web resources (JavaScript, CSS, images…)
To provide a good user experience, LCP should occur within 2.5 seconds of the page first starting to load, or a maximum of 4 seconds to avoid a “bad” score (although between 2.5 and 4 seconds it still “needs improvement”).

First Input Delay
First Input Delay (FID) measures the responsiveness of a page, i.e. the time it takes for a page to react to the user’s first action. When we say first action, we mean any action the user takes on the website, whether it’s clicking or pressing a key. So, as you can imagine, this metric can’t be measured with a tool, but it must collect information from real user interaction.

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When we do a test to measure the Core Web Vitals, this metric cannot be obtained in a real way; it is measured through the Blocking Time (TBT), which is the period between when the first content appears and the page responds. Although this metric is somehow correlated with the FID, it is not real.

It is important to note that page response delays occur primarily while the page is still loading, and when some content is visible but not interactive because the browser is busy loading the rest of the page.


The main factor that influences the First Input Delay metric is:

Using Javascript
To provide a good user experience, pages should have a FID of less than 100 milliseconds or a maximum of 300 milliseconds to avoid a “poor” score (although between 100 and 300 milliseconds it still “needs improvement”).

Cumulative Layout Shift
Cumulative Layout Shift (CLS) is the metric that evaluates the visual stability of a page.


Ultimately, what this metric measures is how long the content continues to scroll despite the page appearing to be fully loaded.

The CLS score is calculated by multiplying the screen share that unexpectedly changed on load by the distance it traveled.

Factors that affect the Cumulative Layout Shift metric are:

Image and video size attributes.
Inserting new content above the already loaded area.
As you can see, this attribute is the easiest to optimize, since you only have to indicate the size attribute in the elements, so that it does not have the effect of rescaling the web, as the page loads.

To provide a good user experience, pages should maintain a CLS of less than 0.1 or a minimum of 0.25 to avoid a “poor” score (although between 0.1 and 0.25 there is still “needs improvement”).

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How we measure Core Web Vitals
Measuring Core Web Vitals can be done through two types of tools:

Field tools: They provide data from user behavior. This measurement is called RUM (Real User Monitoring) and collects data from users who interact with the website.