Is the sprinter conquest all five acts in an hour (12 min per act seems impossible...), or is it every act in 1 hour each (which is easy)?
I'll probably do the same conquests as you, gems should be easy and I think 6xGR55s should doable. I rebirthed a barbarian today and powered to level 50 with lots of bonus xp gear, I'll start gambling shards with her once I get to 70. I have the same problem as you, my comp loads levels really slowly, also slows down significantly with too many other players, so boss mode might be too hard.