The Core Web Vitals is a Google report that evaluates a website's UX, speed, and performance through user data. The term "Core Web Vitals" generally refers to the three metrics used by Google to compare URLs.
- Cumulative Layout Shift (CLS)
- First Input Delay (FID)
- Largest Contentful Paint (LCP)
Largest Contentful Paint
Largest Contentful Paint (LCP) measures the time it takes for the biggest content element to become visible on a user's device viewport from the moment they access the webpage. This element is often a picture or video, according to Google.
LCP measures in seconds. A score of 2.5 seconds or less is the desired score for optimal performance, and anything over 4 seconds is a possible reason for alarm.
First Input Delay (FID)
First Input Delay (FID) is a measure of the time it takes for the browser to respond to the first interaction a user has with the webpage. The interaction can be something like clicking a button, or a link.
FID is measured in milliseconds, with a score of 100 ms or less showing good performance while a score of over 300 ms indicating poor performance.
Cumulative Layout Shift (CLS)
Cumulative Layout Shift (CLS) is the sum of layout shift scores for all unexpected movements on a webpage during use.
This Core Web Vital metric is evaluated by numbers between zero and 1, with a CLS of 0.1 being the target, and a CLS of over 0.25 indicating a need for improvement.
Similar to the LCP, an image or video is often the reason for a poor score.