AI at high is a big cost, at least as much because some of the default aircraft have high Draw call counts. We fixed this for the 3 AI aircraft ( MD-80, Dash8, Cherokee ) and they are 25% of the world wide traffic DB, but default aircraft like the 747 and A320 get used a lot as well.
This adds up. Quickly. High AI is one of the quickest ways to cancel out the benefits of the Draw and SetTexure call reductions we did in Autogen buildings and vegetation.
Having fiddled around with 100% AI a bit, I think you are wiser off sticking at 80% max anyways.
And here is why:
1) Above 80% and the landing pattern gets thrown off, and the AI system injects a plane between 2 that are spaced at 5+ mi, and you get a lot of "wave offs or flybys" by that middle plane; which can be disconcerting because you would never see that at a commercial airport. You still see this at 90%. I haven’t tried 85% or any 1% increments between 80 and 90, but I didn’t notice the flybys at 80%
2) If you are flying the heavy iron, with more than 80% traffic its hard to get a gate, which means no jet way animations, no refueling.
So stick at 80% max.
Fun Factor Alert:
I will say that it’s worth parking on the grass at a busy airport somewhere that you can see both the landing and the takeoff pattern, switching to night time, and switching to 4x time factor. Extra bonus if you can view the terminal at the same time. It’s a hypnotic display at a busy airport and is great as a "screensaver". If you haven’t done this yet, take a couple of minutes and check it out!