What Is Time to First Byte (TTFB)

Short answer

Time to First Byte measures the duration from the browser's request to the first byte of the response arriving. It captures network latency and server processing time.

How it works

The browser sends a request, the server processes it, and the first packet returns. TTFB includes DNS lookup, TLS handshake, connection time, and server generation time.

Example

A user in London requests a page from a server in New York. DNS takes 50ms, TLS takes 100ms, and server processing takes 200ms. The TTFB is approximately 350ms.

Why it matters

TTFB is the foundation of all other load metrics. A slow TTFB delays LCP and every subsequent paint. Aim for a TTFB under 600ms.