For me the A320 has a very stable and equal FPS on all planes even in the external view and everything in Ultra. What is your computer specification? graphics memory?
I have the same problem as Elias Birkholz has it but only with the 320. Changing the Pixel density or graphics quality is not helping at all... Maybe it's a bug or something. For the developers: I am currently using the android version fs 2021 on the xiaomi redmi 9s (6gb ram version)