Core Web Vitals (LCP, INP, CLS)

Google’s three metrics of real user experience: loading (LCP), responsiveness (INP) and visual stability (CLS).

According to web.dev, a page is good when, at the 75th percentile of visits, Largest Contentful Paint is 2.5 s or less, Interaction to Next Paint is 200 ms or less, and Cumulative Layout Shift is 0.1 or less. INP became a stable Core Web Vital in 2024, replacing First Input Delay. Field data from real users decides the result; a fast lab test on a developer laptop proves little.

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Related terms

Event loop: How JavaScript runs one task at a time and queues the rest: callbacks, promises (microtasks), timers and rendering.
Request waterfall: Requests that wait for each other in sequence when they could run in parallel, so load time adds up.
Hydration: The step where JavaScript in the browser attaches to server-rendered HTML and makes it interactive.

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