Time to First Byte (TTFB) measures how long it takes for the browser to receive the first byte of response from the server after making a request. It includes DNS lookup, TCP connection, TLS handshake, and server processing time. A good TTFB is under 800ms.

TTFB directly impacts all subsequent metrics like LCP and FCP. A slow server response delays everything—the browser can't start parsing HTML, downloading resources, or rendering content until TTFB completes. Improving TTFB creates a faster foundation for your entire page load.

Slow TTFB is typically caused by: slow server-side processing, database query delays, no server-side caching, distance between user and server (no CDN), unoptimized backend code, shared hosting resource limits, and cold starts on serverless functions.

To improve TTFB: use a CDN to serve content closer to users, implement server-side caching (Redis, Varnish), optimize database queries, use edge rendering or SSR at the edge, upgrade hosting infrastructure, enable HTTP/2 or HTTP/3, and reduce server-side processing complexity.

Origin TTFB is the aggregate across all pages on your domain from real Chrome users. URL TTFB is specific to a single page. Origin data has more samples and is more stable, while URL data shows page-specific performance but may have insufficient data for low-traffic pages.