I just found a solution to my problem, at least to some extent. When I use Windowed mode instead of Fullscreen, the problem goes away completely. Additionally, when Fullscreen is turned on and I Alt+Tab to another application, the following happens: The application (Chrome, let's say) comes up in front of PA, very reminiscent of Borderless Fullscreen Window, rather than the Fullscreen that you find in other games (for them the entire game gets minimized if Fullscreen is used and I Alt+Tab). The Main Menu can still be hovered over (remember that the game is behind Chrome now) and works smoothly, just as when I turn on Windowed mode. If I bring PA back to the front, the Main Menu becomes laggy again. I don't consider this a full solution, since I would prefer having the game in Fullscreen mode, but it's a lot better than what I had previously. Maybe this could give you guys some clues for figuring out what this could be caused by.
