If you can do 2 heroics per hour i think you are doing just fine. There is always the distance you have to run, no matter how fast you kill. There is always the trash packs in the way, no matter how fast you kill. Some trash is not designed to pull together, as they don't move, slow you down, etc ... then it's just faster to kill smaller packs. I honestly believe - besides a couple of exceptional players who know every single mob with their eyes closed, and who usually outgear the instance a lot, - that the " i can run in it in x time" can be compared to the dps exaggeration.