I've just made an experiment. Before I explain - here's what I have:
- terrain almost 10 x 3 km = 26 760 polygons (21 500 are driveable) - split for parts with indyvidual LOD settings. "Normal" textures (from default XPack and from Haywood Club, mixed together, but no background image (I thought that might been a problem). I don't really need any image with this track.
- 82 400 objects including 76 400 of grass. About 650 objects are collidable.
- 10,5 km roads
My machine is similar to yours (XP SP3, 2Ghz Dual Core, NVidia 8600M GT, 4 Gigs of RAM 677 Mhz - it's a laptop)
Everything was going smoothly till (I guess) the moment I started to put into my project long distance background trees (with LOD at list 2000 and below).
Before that I had FPS like 60/59, in one very crowded place falling to at least 56-57...
Now I have eventually 57 in the whole track, at the place 40-42...
I thought the problem was with that big amount of grass (max LOD 200, mostly 125, in many cases even 65...), so I deleted all of the grass - And had maybe 58 FPS sometime
I was paying a big attention to set low LOD to all objects (and aferwards got sick with looking for single tress with too low LOD...)
I also thought for a while there's some limit that RBR/BTB/or RX Plugin can stand, but after deleting over 76 000 small objects it was just a little better.
The strangest thing is that I see that lower FPS rate from the beginning of the stage, while those high-LOD trees are 5 km or more beyond.
Well, after deleting ALL objects from the track, the stage runs beatifully smooth
I have no idea how it goes then...
I checked original RBR track and other made for RX - great FPS
(Thinking that my computer was broken
I will delete those high-LOD trees and see what happens - later
Your track is very nice, I hope you will find a solution in your case.
I hope you got through my essay, aslo sorry for my bad English