Time to First Byte (TTFB) is a foundational metric for measuring connection setup time and web server responsiveness in both the lab and the field. It helps identify when a web server is too slow to respond to requests. In the case of navigation requests—that is, requests for an HTML document—it precedes every other meaningful loading performance metric.
TTFB is a metric that measures the time between the request for a resource and when the first byte of a response begins to arrive.
TTFB is the sum of the following request phases:
Reducing latency in connection setup time and on the backend will contribute to a lower TTFB.
Because TTFB precedes user-centric metrics such as First Contentful Paint (FCP) and Largest Contentful Paint (LCP), it's recommended that your server responds to navigation requests quickly enough so that the 75th percentile of users experience an FCP within the "good" threshold. As a rough guide, most sites should strive to have Time To First Byte of 0.8 seconds or less. Under 800 ms is good, 800 ms to 1.800 ms was need improvement, and more than 1.800 ms is poor score.