When you're checking an API's speed, it helps to know where your request went. A nearby server and one across the ocean can take the same time to process a request, but the network trip is different.
These are example headers from a response:
x-served-from: Virginia
x-response-time: 14.86ms
x-served-from names the region that handled the request. x-response-time measures time inside the API, up to the point where it sends the response. It doesn't include the trip from your machine and back.
Measure elapsed time from your own client to see what the call costs your application. The headers give you context for that measurement. They don't replace it. The live examples in the IP docs show browser timing alongside the serving region.