The problem is too many objects. Each AI aircraft when placed comes along with a set of ground vehicles. That is, for each aircraft, 7 more objects are plotted. That also includes workers. Due to this the simulator locks the plotting of more objects. If you remove all vehicles in options, you can set max AI traffic option in FSTL (I use FSTL with preference to use FS TRAFFIC models, which are available in the latest FSTL update). I had a lot of trouble balancing everything until I found this solution. In practice you won't even miss these vehicles or boats. I just left some highway traffic for immersion. Today I have no problem with any invisible object from AI or GSX PRO traffic. I operate in SEOUL, KLAX, EGLL, EBBR, KDEN and etc. And I never had any problems again. The problem is the boat traffic and the amount of support vehicles on the ground for each AI aircraft present.