Core Web Vitals

Core Web Vitals is a set of performance and usability statistics that Google has pre-defined. These statistics are essential to ensuring good user experience. They focus on 3 specific UX aspects: page speed, interactivity and visual stability.
 
Google uses these metrics as one of their factors to determine a website’s ranking in their search results.
 

FCP – First Contentful Paint

First Contentful Paint (FCP) times how long it takes for a browser to display the first part of your website. This includes text, images, and video. A good FCP-score is below 1.8 seconds.
 

TBT – Total Blocking Time

Total Blocking Time (TBT) refers to the amount of time between First Contentful Paint (FCP) and Time To Interactive (TTI). In other words, the amount of time between the moment when the first content is displayed, and the moment when users can first interact with the content. A good TTI-score is below 200 milliseconds.
 

LCP – Largest Contentful Paint

Large Contentful Paint (LCP) is similar to FCP, but not entirely the same. Whereas FCP measures the time it takes for the first content to appear, LCP times how long it takes for the majority of the content to be displayed. A good LCP-score is below 2.5 seconds.
 

CLS – Cumulative Layout Shift

Cumulative Layout Shift (CLS) measures web pages’ visual stability, how much the layout of a page moves while it is loading. When the layout moves a lot during the loading process, the user experience is negatively impacted. The measurements include how much of the content moves, and how far it moves across the page. A good CLS-score is below 0.1.
 

Speed Index

By looking at the percentage of content that is displayed at different points in time, Speed Index gives an indication as to how quickly the content of a page loads. A good Speed Index score is below 4.0 seconds.