Could a koukaku ghoul be fast?
I think it is just like you said, but this is just an opinion, as i couldn't find any information about this.

But if we take ghouls as humans, a little child would be slower than a man if both were walking with a 30kg backpack just because the man is stronger.

So a ghoul with koukaku should be able to increase is speed, to the point that would be considered fast, but it would be so much harder because it would not only take him to train his speed but also his strength and stamina.

PS: I consider this to be posible because there are no negative effects regarding to koukako (it is just heavy  and extremely robust)