9-1 has more enemies at once than previous floors I think, and it introduces the aura effects on enemies which has fancy visual elements. It might just be right at the tipping point where it's too much for the phone, if you've already tried altering settings there's really not much you can do.

If you restart the phone from off and reinstall the app maybe you'll get lucky and it'll be better, but if not all you can really do is hope optimization issues eventually make it run better. I'm on a laptop and it certainly gets a bit hotter in intense fights with lots of enemies like 9-1, and then it cools off a bit in 9-2 when there's less enemies to fight at once.