As has been said, the more aircraft (AI or otherwise) you have in your aircraft folder, the longer it takes FS to boot up. Have enough planes and it can get really annoying!
This is why I populate my airfields mostly with static aircraft models rather than AI. The relatively few AI planes in my aircraft folder are planes that might show up anywhere; for example, any USAF base in the classic era might have similar transient traffic like a C-47, C-124, TB-25 or C-131, so types like that take off and land as AI at all the bases, which the mission aircraft unique to each base populate the ramps as static scenery models.
This way I get some aircraft movement at each base, plenty of each base's stationed aircraft scattered around, but my sims boot up in seconds.
Another advantage is that I have an AI plane fire up and taxi for takeoff every ten minutes or so, fly a circuit and land (a bit more movement at big civil airports, which I haven't developed as much as some military bases.) That's enough traffic that I'll see a takeoff and a landing while I get ready to fly and taxi to the runway, but if I'm arriving rather than departing I have a good chance of making an approach and landing without getting cut out of the pattern by the insufferably rude Captain A.I. Pilot.