Let's dive into web performance metrics. In the world of web performance, measuring how fast and responsive a website feels to users is crucial. Google and other major platforms use several Core Web Vitals and related metrics to gauge performance. Here's a breakdown of five essential metrics: FCP, LCP, TTI, INP, and TBT.
1. FCP – First Contentful Paint
Definition: First Contentful Paint measures the time from when the page starts loading to when any part of the page's content (text, image, SVG, etc.) is rendered on the screen.
Why it matters: FCP gives the first indication to users that the page is loading. A fast FCP means users aren't staring at a blank screen for long.
Good score: Less than 1.8 seconds
2. LCP – Largest Contentful Paint
Definition: Largest Contentful Paint tracks the render time of the largest visible element on the page, such as a large image or a block of text.
Why it matters: LCP reflects when the main content of the page has likely loaded and is visible to the user.
Good score: Less than 2.5 seconds
3. TTI – Time to Interactive
Definition: Time to Interactive measures how long it takes for a page to become fully interactive — that is, when a user can reliably click buttons, enter text, or interact with elements without delays.
Why it matters: Even if a page appears visually complete, poor interactivity can ruin the user experience. TTI ensures functionality is ready.
Good score: Less than 5 seconds
4. INP – Interaction to Next Paint
Definition: Interaction to Next Paint measures the latency of user interactions (like clicking a button or typing) across the entire page lifecycle. It considers the worst-case interaction, not just the first one.
Why it matters: INP helps evaluate how responsive a page feels throughout the session. A slow INP means users might experience annoying lags.
Good score: Less than 200 milliseconds
5. TBT – Total Blocking Time
Definition: Total Blocking Time is the sum of all periods between FCP and TTI where the main thread was blocked. These blocks prevent the browser from responding to user input.
Why it matters: TBT directly affects INP and TTI. Long tasks (JavaScript execution, heavy layout calculations) during load can make the page unresponsive.
Good score: Less than 200 milliseconds