The Time to First Byte metric is measured as the time from the start of navigation request until the time that the client receives the first byte of the response from the server. It includes network setup time (SSL, DNS, TCP) as well as server-side processing.
You can find this metric and its evolution in the Speed Metrics tab of Lighthouse’s plugin settings.
The thresholds for fast/average/slow TTFB are 500 ms as the upper limit for fast TTFB and 1500 ms as the lower limit for slow TTFB.
500 msis based on the server responsiveness best practice.
1500 msis based on the requirement for the fast Largest Contentful Paint (LCP) threshold to be <
1800 ms. A server response that exceeds
1500 mswill not, in most cases, meet the fast LCP goal.
The TTFB is the total amount of time spent to receive the first byte of the response once it has been requested.
This metric is one of the key indicators of web performance. Google recommends this value to be between 250ms and 500ms.