Activator performance improvement

Shouldn't close the previous PR early. Turns out I did not have to change much of my design.

Pinging previous trackers
@pranavkm
@halter73 
@rynowak
@ipjohnson
@yishaigalatzer 
Surely, but I think we would want a test for just ServiceProvider. I assume the MVC test covers a lot more than service resolution.