cloud tool for monitoring urls every few minutes

we have a couple of ios / android applications that gets data from json feeds of our website (and a couple of other external services).

Sometimes it happens that the app is very slow at first load (when it has to call a bunch of urls to get all the needed informations), and I'd like to know what is the bottleneck.

My apps call 3 urls in async mode at the same time, but from one of them they get another url for additional informations, and therefore they call it.

So, the calls are:

ASYNC: URL-1, URL-2, URL-3

SYNC: URL3, URL-4

Do you know if there is a cloud tool that allows me to build a test case like this (with mixed sync / async calls)?

I've checked monitor.us and tools like this but no one allows me to build a monitoring that fits my needs.

Thanks a lot for your help / suggestions!