I've tested it with every dagger, and it's only the Ebony Dagger that displays this "unique" trait.
It attacks much slower than the other daggers, at the same attack speed as that of a one-handed sword (or even axe).
It's extremely noticeable if you have another dagger to compare.

I thought it might be a unique trait of the ebony weapon class, so I tested the attack speed of the other ebony weapons but it only seems to be the dagger that attacks slow.
I'm positive this is unintentional, please add it to the laundry list of fixes in the next patch please.

Also, here's a few other bugs I've come across:
-When equipping a silver ring and a silver amulet with a 2% archery enchantment, they both show up as "silver ring/silver ring" or "silver amulet/silver amulet" in the active effects section. I'm not sure what other items/enchantments this carries onto.
-Blessings from the Nine Divines do not show up under active effects, essentially making it impossible to know what some of the effects do. This is a serious issue because these prayer statues are scattered everywhere.
-Extremely easy to do infinite gold trick that I highlighted in another thread.