Core Web Vitals

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Core Web Vitals are metrics that help Google evaluate the user experience of a website. They provide website owners with tangible information that lets them identify weaknesses of their website and optimize its user-friendliness.

Core Web Vitals as means of evaluating the user experience were introduced in 2020. As of summer 2021, the metrics have become official ranking factors as well.

Core Web Vitals

Google categorizes its Core Web Vitals into three different types (loading, interactivity, and visual stability) and rates each of them as either “good”, “needs improvement”, or “poor”.

Core Web Vitals include the following three metrics:

Largest Contentful Paint (LCP)

Largest Contentful Paint is the time that passes between a client requesting a page and the complete rendering of the largest above the fold element and can thus be assigned to the loading category. Google considers anything less than 2.5 seconds to be a good value.

First Input Delay (FID)

First Input Delay describes a page's interactivity and is the time it takes a page to respond to a user's first interaction. Interaction is defined as a click on a link, a button, or a form field, as well as opening a menu. According to Google, a good FID should be less than 100 milliseconds.

Cumulative Layout Shift (CLS)

Cumulative Layout Shift rates the visual stability of a page, i.e. how much each element shifts while loading and without any user input. Layout shifts can lead to unintentional clicks and negatively impact user experience. CLS is given as a number and, according to Google, should not exceed a value of 0.1.

Other Web Vitals

In addition to the Core Web Vitals, there are less critical Web Vitals, used to complement and extend the Core Web Vitals. These vitals include:

  • Time to First Byte (TTFB): TTFB is part of the loading category and describes the time it takes for the server to send the first byte to a user’s browser.
  • First Contentful Paint (FCP): FCP is another loading performance metric and measures the time that passes before a browser shows the first element of a page’s content.
  • Time to Interactive (TTI): TTI belongs to the category of interactivity and measures how much time it takes a page to be fully interactive after being accessed.
  • Total Blocking Time (TBT): TTB also describes interactivity and covers the total time that a page was not interactive, i.e. unable to process user input.

Field data vs lab data

You can use various tools to analyze your website’s Core Web Vitals. These include Google's Pagespeed Insights or the Google Search Console, as well as Google Lighthouse, the Chrome Web Vitals extension, or the Chrome User Experience Report.

Regardless of which tool you use, there are two ways to collect and evaluate Core Web Vitals data: field data and lab data. Both data sets are created in their own way, so the results can be quite different.

Lab data is created in a simulated environment and does not necessarily reflect the actual user experience. For this reason, one good application for lab data is to check the performance of a website during its development.

Field data is generated by the actions of real users and contains data Google collects using its Chrome browser, which requires prior user consent. Field data is suitable, for example, to monitor the effects of previous optimizations.

When comparing lab and field data, it is important to note that there is no lab data available for First Input Delay, since no user interaction takes place. As an alternative, you can evaluate the Time to Interactive in combination with Total Blocking Time, which provides a good indication of the time needed for a page to be usable by the visitor.

Which type of data is better?

Both field data and lab data have their own advantages and disadvantages. That said, lab data is the better choice when it comes to creating and optimizing websites, as you do not have to wait for real user feedback every time you make a change. In addition, field data does not provide meaningful results for websites with low traffic. This is why we also use lab data at Seobility to evaluate the Core Web Vitals.

Importance for SEO

Pagespeed has been an important ranking factor for several years. Consequently, building fast and responsive websites that provide a great user experience has always been an important part of search engine optimization, even before the introduction of Core Web Vitals. The fact that Core Web Vitals are considered an official ranking factor since mid-2021 only reinforces this significance.

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