I found out what's causing this bug, it's the custom vending machines that are part of the weapon-config. The removing of the 'original' vending machines apparently causes the FPS drop around that area. The only fix would be to disable the vending machines, but then the vending machines would become useless. So, I think the best solution for now is to keep the vending machines, but the FPS drop would stay. Maybe an option would be to move the airport to another location, so that area wouldn't be used as much, and the FPS drop wouldn't affect too many people?