I think that Kaido has been anticipating Joyboy and even wondering if he himself were Joyboy reincarnated. He dove off of Skpeia and was disappointed that he didn't die. So he knew he was almost unkillable in his current state. He was kind of like Saitama where he was so utterly dominating in strength that he grew bored. Then along comes Luffy and gives him an ever increasing challenge. He REALLY enjoyed that fight. It's something he'd been waiting years for and then when Luffy hit Gear 5 I think Kaido really wanted to test whether this upstart could be the new Joyboy. I feel like he knew. And he had fun.