I reckon this is a result of the infamous 'cliff' syndrome - where the FS scenery has been either flattened or otherwise height adjusted by the add-on airport scenery to make it more accurate.
So the Cessna will be at MS Chivenor elevation, and your Jet Provost is at the height UK2000 reckon it ...
Don't worry, Tony - that's the basic frequency of your RAM as set by the manufacturer in the controller chip on your modules. It is what the system refers to, to find out how to run your memory.
So - take your basic memory speed - 333MHz.
It's DDR - double data rate - so multiply that base ...
Yeah - I tried that, too, mate - it made no difference at all. I also tried sliding the minimum FPS setting from 10fps (the lowest it will go ) to unlimited and there was still no difference.
I think it would take a long time to suss that out, so I'll probably not bother!
I forgot to add I was using "Fair Weather" with Trilinear and Anti-aliasing set.
When I disabled anti-aliasing and trilinear (ie "none" setting for texture filtering) it made no difference at all to the framerate - so something is most definitely wrong somewhere. The picture ...