I truly don't wanna powerscale King now when we have zero information about the combat content of this chapter lol. We were just told that King used a named attack and Zoro used this.
I'll wait till I see how the fight goes.
But for the most part: we can agree that regardless of how fast the speed form is, King ain't gonna tag Katakuri. So what's the point in even using the speed form against him? King can just keep his flames up and fight Katakuri while taking everything he has. It's not like King does not have attacks in his defense form (Zoan/Base).
As I said, it will be a battle of attrition - and I'll bet on the Ancient Zoan to last it out 9 times out of 10.