Time to first byte (TTFB)
Also called: TTFB
How long the server takes to start sending a response after the request.
Definition
A slow TTFB points to the server or backend, not the page size. In DevTools it appears as the 'Waiting for server response' part of a request.
In practice
The recommendations API waits 3 seconds before sending anything; that's a backend issue to report.