Time to First Byte (TTFB)

Time to First Byte spans DNS resolution, connection and TLS setup, the request, and server processing, ending when the first response byte reaches the browser. It is exposed through the Navigation Timing API as responseStart relative to the navigation's start.

TTFB is not a Core Web Vital but bounds everything after it: no pixel can render before the first byte lands. Values under roughly 800 ms are commonly treated as acceptable in field data; sustained high TTFB points at origin processing, cold caches, or missing edge delivery.