Hi,
I noticed on my computer that the startuptime is dependent of the amount of installed planes.
Planes I don't use very often are either deinstalled or the folders are put out of my airplane folder to a folder named UNUSED AIRPLANES.
So if I want to fly one shifting back the folder will make them available quickly.
If uninstalling airplanes I also check the fsx.cfg for remains of gauges etc (section: [Trusted)).
Another item to ceck is the logbook file. I use FSXLog.exe for this task.
Good luck!