TTFB (Time to First Byte)
Time to First Byte (TTFB) measures how long a browser waits for the very first byte of the server response after requesting a page, and so reflects server and network speed before rendering can even begin.
In practice
A slow TTFB – often caused by slow hosting, unoptimised server-side code or a missing caching layer – handicaps every subsequent performance metric from the outset, because nothing can render faster than the server responds. web.dev names 0.8 seconds or less as the target; TTFB is not itself a Core Web Vital but a diagnostic metric, and a poor one pushes LCP up above all.