Definition
TTFB (Time to First Byte) is the time between the moment a browser or crawler requests a page and the moment the first byte of the response arrives. The measurement includes the DNS lookup, establishing the connection (TCP and TLS), the time the server needs to build the page and the return trip across the network. Google considers a TTFB under 800 milliseconds good and above 1.8 seconds poor.
TTFB is not a Core Web Vital, but it is the foundation beneath them: every millisecond the server delays pushes Largest Contentful Paint back directly. A page with a TTFB of 1.5 seconds almost never meets the 2.5-second LCP threshold, no matter how well the images are optimised. That is why TTFB is the first number to look at on a slow-loading site.
The causes usually sit on the server side. A CMS that rebuilds every page from the database on every request (without page or object caching), slow database queries, overloaded shared hosting, plugins or apps that call external APIs on every request, and a hosting location far from your visitors. Redirect chains count too: every redirect is an extra round trip before the first byte of the final page arrives.
Fixes in order of impact: full-page caching or static generation so the server serves a ready-made page, a CDN that caches pages close to the visitor, HTTP/2 or HTTP/3, fast DNS, a better hosting tier, and hunting down slow queries and plugins. On headless and Next.js sites, the difference between static, cached and per-request rendered pages is often the entire TTFB.
For AI crawlers, TTFB weighs more heavily than for Google. GPTBot, ClaudeBot and PerplexityBot fetch pages with shorter timeouts and give up sooner on a slow server; a page that does not respond within that time is not read and therefore not cited. Traze measures the TTFB of every published page and of the site as a whole in the technical scan and flags it when response time stands in the way of citability.