Given all of the above types of performance metrics, it’s probably clear that no single metric is enough to capture all of the page’s performance characteristics. What users perceive as a “fast” page depends on several metrics that try to capture it over time: User experience Metrics Is this happening? TTFB, FP, FCP Is it functional? FMP, SI, LCP Is it useful? TTI, TBT, FCI, FID Is it delightful? CLS Time To First Byte (TTFB) measures the time from when the user sends an request to when the first byte of the page is receiv by the user’s browser.
First Paint (FP) informs about the time when the first pixel appears on the screen after the user goes to the website. First Contentful Paint (FCP) measures the time from when the page starts loading to when any part of the page’s content is render on the screen. First Meaningful Paint (FMP) is the time in seconds between the user initiating page load and the page displaying the main content. Spe Index (SI) measures how quickly content is visually display on page load. Large Content Paint (LCP) measures the time from when the page starts to load until the largest block of text or image element is render on the screen.
Time To Interactive (TTI) measures the time from when the page starts loading until the page is visually render, its initial scripts (if any) have been load, and it is able to quickly and reliably respond to user input. Total Blocking Time (TBT) measures the total time between FCP and TTI that the main thread was block long enough to not respond to BU Leads input. First CPU Idle (FCI) measures how long the page becomes minimally interactive, : most of the user interface elements on the screen are interactive, and the page responds reasonably to most of the data enter by the user at a given time.