The “fix” in 1.21.60 has made it worse, they now teleport to Y-64 if there is room, else they teleport to Y-80 below build limit.
I got to sign up for the Minecraft preview today iOS. I’ll do some more testing after work but I did get 10 minutes on 1.19.10.23 and is significantly better performance. Even setting render distance to 18 instead of default 10 worked, though flying far I could get to chunks still to be loaded but even they loaded a lot quicker.
I also copied my world which I have been working on for months into preview folder and it was nice seeing everything loading as it should be.
I’ll do some more testing later
They are going to Y-64 if there is space, test a bee, go below build limit you find happily flying around. Not good!