Sounds like a servo problem to me too. Is one of the servos older than the other one? Errr, maybe one of the servos has a slightly messed up wire (which is not allowing it to get all of its potential voltage). The damage might or might not be visable.

What kind of speed difference are we talking about here? If its a lot you might be able to place both of them side by side (with no linkage connected) and see if you can tell the difference. That would eliminate most of your mechanical problems if you could tell the difference.