The games poor performance is an issue I'm painfully aware of and something I'm currently trying to fix. It's Mostly because of bad coding practices on my part and I'm currently refactoring a lot of the AI code to try to make it scale better.
Due to the aircraft's use of somewhat "realistic" physics, the aircraft isn't exactly stable in the air and will tend to drift down due to an unbalance of forces. I've experimented with an auto trim feature to help alleviate this but it didn't really work well so I scrapped it. I will be sure to try and implement a proper one in the future. Thanks for the feedback!