Ok... I carried out some tests..
First; I experimented with taking out planes. This had almost no bearing at all. Startups were the same.
With each startup, I rebooted the computer so RAM was refreshed or empty/clear. I waited till the computer OS was stable after the restart before starting P3D. I then clicked on the P3 icon and started the timer.
Differences were only 10 seconds, if that. I believe it makes no difference on how many planes you have in your Airplanes folder. Maybe if one had 200 planes, but in this case, I only have about 60 aircraft.
Start times were always 3:30 to 3:40, with one at 4:00 mins, from click to tarmac.
I was also booting up into an ORBX rich area' a payware airport scenery, Diamond Point, with some animations turned off and scenery settings (general) around standard half way points on the sliders, water at 25% setting, normal clouds, mostly stock settings, but scenery effects and scenery detail set to full. (Not terrain textures). Autogen also at 50%.
Now, to change things, I booted up into stock Montgomery airfield in San Diego and startup was almost 1 min quicker, boot up time; 2:50. So I believe scenery has everything to do with it. I back this up with having watched the loading screen scripts; 'loading terrain data' lasted to the 70% mark. Plane and autogen and others were loaded in seconds. Most of the loadup is the terrain data.
So that is that. Good to know. My hunger for ORBX rich realism was the culprit, not the amount of planes in the airplanes folder. Good to know.