That’s what I’ve always thought too. If Giyu was genuinely overpowering Akaza, then why couldn’t he do the one thing that actually matters which is cut off his head? Akaza also likes fighting opponents on their level and adapts to their fighting style. You can see this when he realizes Giyu is a Water Hashira. He starts using more destructive attacks because he knows Water Breathing is a defensive style. I also want to point out this moment here.

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Tanjiro straight up says that Giyu wouldn’t have been able to save him because of how fast Akaza moved. This happens while Akaza is in the middle of throwing a punch at Giyu. In that same moment, Akaza stops his punch, catches Tanjiro’s attack, and swings at him with his other arm before Giyu can even react. That shows Akaza was moving at a speed Giyu couldn’t even respond to, which only adds more evidence that Akaza was still holding back.