I have noticed incorrect and inconsistent behaviour of the HttpClient class when making Web Api calls.
When making multiple calls in parallel to an IP address which is not reachable, then the timeout is not honoured (sometimes). If you only send one request the timeout is honoured
I have raised a bug on the xamarin bugzilla site, and added a simple demo and instructions on how to replicate it.
Can someone from Xamarin support have a look at it.