Slow web api start IIS

I have 2 web api :

• A : serve my front-end answer (called by user)
• B : manage some specific feature. This one can only be called by A.

So if I resume I have the following architecture:

User -> A <-> B

B is not really often called and I notice that the first call takes ages to proceed, but after the first call is really fast (at least fast as expected to be).

How can I prevent this to happen (especially I don't want B to be idle or have a slow start). I tried to fix this issue by setting the ApplicationPool used by B in "AlwaysRunning" start mode but it doesn't fix the issue. A cheap solution is to "precall" B when I think A will call B but I think is really weird and I want to avoid calling for nothing...

Thanks for the help.