AI craft can certainly consume CPU resources when they are enabled... depending on the time of day, some larger airports can get very busy with AI craft flying in/out and moving around... i've seen some airports with 20 or more AI all moving at once... sometimes they'll get hung up somewhere and back up a lot more than just 20... i used to make a game of it to find the one that was hung and give it a sharp kick in the backside (via the property tree) to send it on its way... once that one was gone, things worked smoothly for a while in most cases... sometimes another one will get hung up... i've seen them take over an hour to have all the departing craft finally get gone...
these ones being hung are likely not your problem, though... i suspect there were just numerous ones in the area that your sim was moving about... the area is, IIRC, 100km or 200km... i forget as it has been so long since i played this ""game"" of watching and booting the AI when needed...