Other Web Vitals
These other Web Vitals often serve as proxy or supplemental metrics for the Core Web Vitals, to help capture a larger part of the experience or to aid in diagnosing a specific issue.
Time To First Byte (TTFB): this audit fails when the browser waits more than 600 ms for the server to respond to the main document request. Users dislike when pages take a long time to load.
First Contentful Paint (FCP): measures the time from when the page starts loading to when any part of the page's content is rendered on the screen.
Time To Interactive (TTI): measures the time from when the page starts loading to when its main sub-resources have loaded and it is capable of reliably responding to user input quickly.
Total Blocking Time (TBT): measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked for long enough to prevent input responsiveness. The main thread is considered "blocked" any time there's a task thet runs more than 50 milliseconds.
First Meaningful Paint (FMP): deprecated - measures when the primary content of a page is visible to the user. The raw score for FMP is the time in seconds between the user initiating the page load and the page rendering the primary above-the-fold content.
Speed Index (SI): measures how quickly content is visually displayed during page load. Lighthouse first captures a video of the page loading in the browser and computes the visual progression between frames.
First CPU Idle (FCI): deprecated - measures how long it takes a page to become minimally interactive. A page is considered minimally interactive when most—but not necessarily all—UI elements on the screen are interactive, and the page responds, on average, to most user input in a reasonable amount of time.