I have a very extensive collection of AI aircraft in my FSX installation, which consists of just about every WOAI package in existence, plus Ultimate Traffic 2 and a tons of others like MAIW, ultimate GA, and everything in between. As a result, my AI folder takes up a sizable amount of hard drive space (12 gigs). To be honest, I'm really not concerned with disk space since I have it installed on a 1TB Western Digital Caviar drive which is less than half full, but I am curious on how this amount of AI aircraft impacts FSX performance in terms of CPU cycles. Does FSX scan the entire AI folder during initial load or on a continuous basis when in operation, or does it only poll the folder for AI that is active in FSX's traffic toolbox? Does it make sense to take the time to delete AI aircraft paints/models no longer "in service"?I have deleted older paints/models in the past, in drips and drabs, but if I was to find out that deleting obsolete AI can impact performance in a significant way, I'm inclined to run though the folders and delete some more. I wanted to know what the experts have to say about this.

Does FSX scan the entire AI folder during initial load

Yes it does looking for duplicate names.

Does it make sense to take the time to delete AI aircraft paints/models no longer "in service"?

If you mean that you have removed the flightplan(s), then yes.