In 2020 the Core Web Vitals are three metrics: Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS).
Each metric measures a different aspect of user experience: LCP measures perceived load speed and marks the point in the page load timeline when the page's main content has likely loaded.
FID measures responsiveness and quantifies the experience users feel when trying to first interact with the page; and CLS measures visual stability and quantifies the amount of unexpected layout shift of visible page content.
Each Core Web Vitals metric has associated thresholds, which categorize performance as either "good", "needs improvement", or "poor"
Largest Contentful Paint:
- Good: <2500ms
- Poor: >4000ms
- Percentile: 75
First Input Delay:
- Good: <100ms
- Poor: >300ms
- Percentile: 75
Cumulative Layout Shift:
- Good: <0.1
- Poor: >0.25
- Percentile: 75
Additionally, to classify the overall performance of a page or site, google uses the 75th percentile value of all page views to that page or site. In other words, if at least 75 percent of page views to a site meet the "good" threshold, the site is classified as having "good" performance for that metric. Conversely, if at least 25 percent of page views meet the "poor" threshold, the site is classified as having "poor" performance. So, for example, a 75th percentile LCP of 2 seconds is classified as "good", while a 75th percentile LCP of 5 seconds is classified as "poor".
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