I also have this issue and I'm using a low end mobile. Maybe it's because some mobile devices are struggling to load experimental worlds because of the height limit change. Now your device has to render or generate more blocks.
The device I'm using is a HUAWEI MediaPad T3 7 (BG2-U01)
Turns out it's not the hight limit change causing this. I tried the 1.18.0.20 Beta where they put the caves and cliffs features outside of experimental features including the hight limit change. I tested it and the world loaded but it's very laggy.