Zoro's injuries are healed but he's still exhausted from his previous battles. Plus, Zoro's been eating attacks that can overpower his own attacks—his own attacks have a higher AP than post Udon Luffy, who hits much harder than WCI Luffy, who in turn hits way harder than Katakuri. He's eating attacks that are several levels above anything Kuri could dish out bar his most powerful move. So of course he'd look like he's losing breath.
Zoro's attacks are far more lethal than WCI and Post Udon Luffy but he doesn't hit harder. In a direct clash Boundman would still overpower him imo.
I'd say multiple attacks at once won't be an issue for Zoro but back to back attacks are going to be a problem. Katakuri is not faster than Marco or King. I don't think fighting Snakeman is a better speed feat than keeping up with Kizaru.
Katakuri can use his Awakening to trap Zoro from the ground, benefitting from the sticky properties of his Mochi while at the same time attacking Zoro from multiple directions like that
Marco's physically faster than Katakuri and Luffy and the same applies to King by extention—as he was keeping up with Marco in base. I would agree FS+his speed might give him a slight edge over them. Kuri's definitely giving Zoro a hard time; but Zoro's more of an endurance monster than Luffy and can fight, on average, at a much higher level than WCI Luffy. So he can fight Katakuri for much longer while not receiving anywhere near as much damage as WCI Luffy. Once Katakuri's FS shows any signs of wearing down; he's getting two or three shotted. It's not easy but Zoro's fully equipped to put down Kuri.
Overall I think Katakuri puts on the pressure on Zoro, tags him several times with damage that build up over time, dodges most of his slashes that tax Zoro's stamina a lot and ultimately takes home the W.