Hi Hans,
No, it's an undiscovered bug. What you are suggesting is true to a point and on lesser PC's. I have found you can have fast loading aircraft textures, then at some point, maybe weeks, maybe months, BOOM, suddenly you get the slow loading textures I described. Same hardware, same scenery files, same everything, the loading just suddenly becomes slow and stays slow.
I have then found a fix to get the textures loading quickly again without making any change or converting of the texture files themselves. Most modern PC's should comfortably be able to load any aircraft textures quickly even 32bit textures. T'is a bug.
I want to get some more information and do some more testing before I post any fixes to make sure the fix will work for everyone.