Stuttering in Prepar3D

I enjoy flying the F-16 fast and at treetop level... who wouldn't!
But at all levels I was getting stuttering and slowdowns. The stuttering being much more immersion breaking that any slowdown.
Anyway, cutting to the chase after hours and hours and hours over many nights, I worked out the single most important graphic setting (for my rig). And it's one I would NEVER have guessed.

The VERY FIRST setting in the graphics menu.

And it was totally not what I expected. Putting "Level of detail radius" to the lowest setting gave me the best frame rate. Nothing else made much of a difference.
Also, GPU memory usage never went over 4.5 or 5 out of 7.8 (?) gb available no matter what I threw at it.

As you can see, my other settings are pretty well maxed out. Higher Anti-aliasing and texture size settings didn't make much difference to the visuals. I did have 'use high res tetures' selected on the first screen however, as per a forum suggestion.