Time to First Byte (TTFB).

Time to First Byte (TTFB) measures the duration from when a browser sends an HTTP request to when it receives the first byte of the server's response. It is the sum of network latency (round-trip time to the server), server processing time (generating the HTML), and any middleware processing time.

TTFB and Core Web Vitals

Google recommends a TTFB under 800ms. While TTFB is not a direct Core Web Vitals metric, it is the foundation of all subsequent loading metrics—a slow TTFB delays the start of every download, pushing LCP and FCP later. A TTFB above 800ms almost always prevents achieving a 'Good' LCP score (under 2.5s).

Reducing TTFB

Key TTFB optimisation strategies: use a CDN to serve cached responses from edge nodes near the user; implement server-side caching to avoid regenerating HTML on every request; optimise database queries and server-side code; upgrade hosting infrastructure; and enable HTTP/2 or HTTP/3 to reduce connection overhead. For Astro and Next.js SSR sites, ensuring frequently-requested routes are cached at the edge is the highest-impact TTFB improvement.