Havedone this route a couple of times, though not to LAS. Last time arrived at PHL 30 minutes early and was through Immigration and Customs, got my hire car(was Fly Drive this time) and on the I 95 before the scheduled arrival time. Found the staff in PHL were very organised when you enter the Immigration Hall and allocate you a line to get in.
As said previously, forget the doomsayers on here, you will be OK subject to normal operating methods and normal weather etc.