The fps drop affect me on multiplayer worlds aswell so it can just be worlds hosted by you. Im on a samsung Galaxy a40 ( about 3 months old).
No I have only had it since 1.16 betas
I have also noticed it is much worse when moving around and the faster im moving the worse it is so it is probably chunk loading/unloading.
Definitely not because of the I-pad being old. My phone is brand new and before the update my game would run smooth at 16 chunk render distance and max graphics settings. Now I have fancy graphics off and a 6 chunk render distance and I get FPS drops regularly. Im on a Samsung Galaxy A40.
The lag does affect me in the nether