Server Response Time

Server response time (TTFB) is how long the browser waits, after sending its request, for the first byte of the response to arrive; the heaviest link in the loading chain and the only one shortened by optimizing code and the database.

TTFB clarifies the boundary of responsibility: everything before it is the network and the user's operator, everything after it is your code, so this number is the first to say where the slowness lives. But it has two competing definitions often blurred: one from navigation start to first byte (which also contains DNS and connection) and one covering only the server's wait, from the request arriving to the first response byte. Google's standard bands — good up to 800 ms, needs-improvement up to 1800 ms — belong to the first definition, so applying them to “server wait only” makes a card artificially optimistic. A high TTFB usually signals a slow query, heavy server-side rendering or a missing cache, and being skewed it must be read at a percentile, not the average.