Ive run 3 external hds off a Belkin wall-powered hub and gotten the same speed as a direct connection, however my hard drives were all wall-powered as well. As a rule of thumb, it's definitely better to get powered USB hubs for any hard disk usage or heavy data access (bigger thumb drives, etc), while self/computer powered hubs are usually for mouse/keyboard and generally device inputs. There is no downside as far as I am aware of using a wall-powered hub, so if you have the extra cash, its always a good investment to go with external power. I personally use an old model of Belkin's hubs, and its served me well for almost 3 years now.
tl;dr: Drawing usb power is bad for lots of data transfer, grab an externally/wall powered hub and you should be fine. Try and get the hard drives themselves powered too -- the less stress on the hub, the better.