King has impressive AP. His flying slashes overpowered Zoro and sent him flying. Zoro deflected Kaido's flying slashes like they were nothing. King's imperial arrow overpowered Zoro's ultra tiger hunt and send him flying through the wall and the giant skull. Big Mom's lightning sword attack couldn't even break the floor.
King's beak attack destroy part of the island.
FS is grossly overrated. Especially when you have to stay calm to use it. Katakuri was hit by base Luffy whose was veteran level at the time.
King doesn't become weak when he is or isn't using a power. His defense or durability is weaker when not using flames. He goes from having a stronger defense then Kaido to a weaker one. Zoro didn't say King was slow when he's using his flame. Zoro is incredibly fast one and it has one of the fastest attacks you seen in the entire manga. He has incredibly fast reaction time.