An aspect of reaction and combat speed that I never see people talk about is the consistency of said speed.
Base Luffy without FS was almost untouchable against Katakuri when focused entirely on dodging, but got beaten to hell when he tried fighting back with FS aka, he can't kept up the dodging at all times. Luffy with FS was capable of landing hits against Katakuri, but very rarely. Luffy needed Snakeman to land blows consistently.
So when it comes to Kaido and Kizaru, the difference is that Kizaru's reaction time is consistent enough to make him untouchable to Snakeman, while Kaido can't keep up with such reaction speed and gets hit even by Base Luffy.