Time To First Byte (TTFB) measures how long after a request before the first byte of the response arrives. It captures DNS lookup, TCP/TLS handshake, request routing, and server processing — a comprehensive backend health signal.
TTFB is the floor for every other loading metric: LCP can never be better than TTFB plus the rendering time. A TTFB above 800 ms almost guarantees a poor LCP no matter how well you optimise the front end.
Factors that affect TTFB: origin server response time, distance to the user (mitigated by CDNs and edge rendering), database query speed, and SSR rendering cost. Measuring TTFB regionally exposes geographic gaps that an aggregate average hides.
TTFB is often the first thing to fix when LCP is bad. Caching at the edge, putting static HTML on a CDN, or moving to edge functions can cut TTFB from 1500 ms to under 200 ms — and pull LCP into the green automatically.
See it in the product: Performance monitoring.