Where is the data cache stored in ASP.NET

Question

I want to be specific here, please read the question before marking it duplicate.

I have a case where I would have hit a webservice to gather user information and perform a search query on all the users, the user count expected here is around half a million, as a result I thought adding the users to HttpRuntime.Cache the first time and querying from the cached results would do us good.

But my questions are -

1. What is the physical location of the run time cache?
2. Is adding half a million users to cache going to affect the flow?
3. If yes, What is the better way do this?

Kindly revert for more information.