When the server receives the request, the server immediately begins fulfilling it. The server runs all the code that is needed for the request and makes any required calls databases, external APIs, files on the server, or other servers, all of which take time. So if you see a long TTFB on your requests, especially your XMLHttpRequests(XHR)s, then you may want to look at your back-end code or see if you can reduce calls to the database or external APIs.