The wait covers more than the server thinking. It includes the DNS lookup, the connection, the TLS handshake, any redirects along the way, and only then the time the server spends building the response. A redirect chain can add a whole round trip before the real request has even been made.
It matters because nothing can be painted before the first byte arrives, so TTFB is a floor under LCP. If TTFB is 1.5 seconds, no amount of image optimisation gets LCP under the 2.5 second boundary. Fix the server response first, then the content it delivers.
Updated August 2026