Zoro attack speed is fast, i agree, but mobility so far is normal.
Ofcourse with that size, it can blown people away.
Well, we only see him fighting with a normal sized human, so he dont have enough DC to show. But he can produce infinity armor that can attack Pica in every direction, from lower body to upper body. And he just need to clapping.
Attacking Luffy and Nami, that is his way of fighting. Made the armor do the direct attack.
While Zoro might not have great mobility, it is worth noting his mobility is underrated as he was shown comparable to G2 Luffy when both speed blitzed Holdem and Speed and at times he has matched Sanji's running mobility particularly against Pacifista in Sabody return arc.
I am not suggesting Pica has overall superior combat ability than Cracker, Cracker surely has more skill and talent.
But it is also a matter of match ups, Cracker doesn't seem that mobile of a fighter at all and he relies on his defense heavily and hasn't shown offense to be able to deal with what is pretty much a moving mountain, without massive punches. It is pretty much emphasized by Bartolomeo where his barrier has shown to be basically unbreakable yet he said Picas attacks are too big to defend against.
Just how many biscuits is he gonna make when Pica can manipulate the very ground, throw punches that level towns, summon massive city block sized spikes from out of blue?
Based on available feats, there just isn't a logical means through which he can defeat Pica.
Though there might be people out there like Luffy with Gear 4th (who can defeat Pica) yet would loose to Cracker.
Again match up issues