Adrian39, I think after a while, most users of FSX and ilk will experience your problem. Many of them mentioned are very good, but I use a combination of those suggestions. Namely, I sort my aircraft into folders "Military Props",Military Jets, Civilian...General Aviation...Helicopter...you get the idea. Then the Category I wish to fly for extended time, couple of days, whatever, I rename the default Aircraft folder to AC, or something else and rename the category folder I wish to fly to "Aircraft".. Reduced loading times tremendously. If that's too cumbersome, the easiest way to speed up loading, of all the liveries you DON'T want to load, go to the corresponding entry in the aircraft config file and just place an * or ? in the panel= entry. That aircraft will not load. I limit having default aircraft in the folder by placing commonly used panels in the FSX/gauges folder and move whole sound folders into the aircraft.... I limit the use of the "alias" routine. Decreasing the number of aircraft in the Aircraft folder definitely speeds FSX initialization. Many good suggestions out there.
Good luck..Let us know how it works out..........Terry