As well as significant performance problems, there's a core bug with the autoplanner which means flights won't be scheduled properly late game (in my case, 12 small stands and 24 medium stands; didn't even get to large stands!).

It is possible to manually schedule flights, but then you'll need to manually schedule hundreds of flights per game day every game day.

More here:

https://steamcommunity.com/app/673610/discussions/0/3829791007700283290/
https://steamcommunity.com/app/673610/discussions/0/598534669681575393/
https://www.reddit.com/r/airportceo/comments/rv1im2/autoplanner_not_working_correctly/
https://www.reddit.com/r/airportceo/comments/1aihdsh/auto_planner_not_working_on_large_stands/