Actually, the more innocent people you kill with it the stronger it gets.
The ACTUAL reason for this is because it is a DAEDRIC ARTIFACT!! I don't know which daedric prince though.
It was bugged prior to 1.9, and used one -handed perks. Must have left the speed. It will have lower damage because it can't be tempered at the grindstone, but that's a trade of against infinite charges.
If you want damage and speed, look for the Longhammer (I won't spoiler the location).