Vs Big Mom
After Kaido knocks Luffy down with Thunder Bagua, Big Mom hurries to blitz him with Heavenly Fire:
When she's about to make contact with Luffy, the Grandmaster begins his counterattack:
As we can see from this scan, there's considerable distance between Zoro and Big Mom/Luffy:
Faster than Big Mom or Luffy could perceive (let alone react), he blitzes Big Mom and cleaves Prometheus in two:
Prometheus is left temporarily unable to fuse back together:
Conclusion
This is an amazing speed feat for Zoro. He blitzed a Yonkou in the middle of their attack from point blank range with a considerable starting distance. The Grandmaster's attack speed once again impresses. This is one of the best speed feats in the series given that Big Mom's reaction speed should scale to other top tiers.