Why does everyone love Zweihander?

My first time playing the game so i suck, but i've seen people praising this sword all the time, saying how its OP, but ive been using it and its not that good.

It has nice range and staggers enemies, but its just soooo slow, its useless. Surely im doing something wrong right? Is there a way i can make it swing faster? I upgraded it a couple times already.