You haven't kept many Plethodontids have you :P. They are all like that. I'm glad to hear they are doing well - I look forward to meeting them.
Well, I have kept Bolitoglossa dofleini and it never moved that quick. Well the tongue did anyway. I have also kept Plethodon cinereus on occasion, I even caught one the other day and they don't move as nearly as fast as these little Eurycea lucifuga.