If you see in this panel, Zoro is behind Prometheus and Big Mom. Zoro didn't cut Prometheus with close ranged attack. Zoro launched a long ranged attack at Prometheus because his feet cannot catch up to Big Mom.
Zoro's long ranged attack might be faster than Big Mom close ranged Prometheus attack which is not that bad.
Nope, first of all ranged attacks are even slower (so this is worse for Big Mom). You can see Zoro used an Iai technique, the motion lines around his body are indicating movement.
She was
right ontop of Luffy and initiated her attack
before Zoro started his and still got blitzed by him. She got outsped badly.
There's also this right here, where he is able to carry and move Luffy out of the way. She missed badly.
All in all her attack speed is absolutely nothing for Zoro, his top end moves are many times faster (as demonstrated when he blitzed her after she had a head start with her attack).
Like I said, the answer to this thread was already given to us in the manga as directly as we can get it. If this isn't enough to answer your question than there's no point to this thread.