Seems like the speed difference is probably caused by individual attributes, was the dexterity set really high?
EDIT:
I was talking to the guy who tested it, because he may have given him really high dexterity, as I usually do to my adventurers. Also, I like the [CAN_LATCH] on the tail, because then it's like a tail whip where it wraps around the enemy.
I just tested it, since I found some extra time. The tail-slap is really only useful when you hit, so I don't think it's overpowered. I was able to take on a vampire as my first enemy, but only because it was in the middle of a crowded room of people the vamp decided to hit first. (as a side note, it scared the shit out of me because I didn't really expect that the Cave-fish woman ranger was actually a vampire. And I got spammed with about 10 or more pages of human hair amulets and human bone rings falling off of limbs. Yes, the Human hair jewelry was the tip-off.) I decided to test my naga against more undead, to see how slow they would die. It didn't take very long, since the tomb was full of corpses. Then, I decided to make a master kicker, to try and emphasize the kicking move.
So, I don't think that the Nagas are overpowered, except for the innate swimming. That's the biggest advantage they have over the other races, but since they all really have some advantage (better equipment, faster movement, more settlements) I think it's pretty nice.