Core Web Vitals introduced
Google defined a standard set of user-experience metrics — LCP, FID and CLS — as the Core Web Vitals.
What changed
Google published Core Web Vitals as a common set of field metrics covering loading, interactivity and visual stability: Largest Contentful Paint, First Input Delay and Cumulative Layout Shift. Thresholds were defined for good, needs improvement and poor, and the metrics were wired into Lighthouse, PageSpeed Insights, the Chrome UX Report and Search Console. The set was explicitly designed to evolve annually, which it later did when INP replaced FID.
Timing
Rollout began 5 May 2020. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.