AI traffic is scenery - just movable scenery.
Many, many of us have 'static' aircraft at some airports. Usually special aircraft for some purposes.
The way we make that work is writing a flightplan which includes only one movement per week - out and back about 2 am to a nearby airport.
Yes, AI traffic will be generated at the gate and sit there until the next flight for that AI aircraft starts - even if it is two or three days away.
FS does not present a moving world.
It presents the world at a specific time, with all the traffic databases read, and any aircraft which are scheduled to be within about 120nm of your aircraft at the time the FS session starts - they are created. Those in the air, and if there is available parking on the ground. The FS world starts moving at that point.
As far as FPS, aircraft in the air take resources - even aircraft you cannot see.
But they take far less resources than the aircraft you can see. It really does not matter is the aircraft you can see is on the ground or in the air - basically the same impact on FPS occurs. Each time the aircraft is presented in the frame, the polygons are drawn and textured.
That's why good AI aircraft use LOD - Levels of Detail - basically progressively less detailed models the farther your eyepoint is from the aircraft. That gives you a visual, but taking less resources.
If you are talking about converting your flightplans to static plans for airports - it's a huge task if you are concerned with more than a couple airports. Good luck.
@ PawPaw's house - near KADS, Addison, Texas, USA