It suddenly dawned on me:
I was building a small mission from scratch in FMB on the Malta map. After I added an extra flight of a/c, I suddenly noticed EXTREME frame drop that wasn't there before.whenever zooming to the horizon with FOV mod. This has been a problem I've noticed many times. We're talking from 30-60 fps to 1-2 fps; always when zooming my view on the horizon.
Here's a typical high zoom view from a static camera:

On a hunch I renamed my MAPMODS folder so it loads very highest in my MOD folder. I have to mention I have a very large _tex folder: clouds, HD land, Bender's water, all the landscape related mats etc. etc.. Now Maps/_tex has the highest loading priority.
Now the stutter on the horizon is gone!
This HAS to be an issue related to memory allocation on mission load. The question is, which specific files were affecting frame rate? Map textures in general? Road. cloud, sky textures and all the various terrain and sky.mat files? Do a/c and objects always have memory priority regardless of their load order? Not sure yet.
This weekend I'm going to try an experiment with an "overloaded" air.ini file, and see if I still keep the consistent frame rate with MAPMODS/maps/_tex still loading highest. Theoretically, the extra air.ini entries that are displacing memory should in turn be displaced by prioritized map textures. I presume the game will discard air.ini fmds that aren't specifically in the mission, but only testing will answer that question.
This should give a better idea of how to deal with performance issues that aren't related to raw CPU power, and the number of vertices in a scene the CPU has to process, another topic for discussion.