The problem is the descr_grass.txt file. RTR, SPQR and several other mods use a modified descr_grass.txt file that increases the grass blades 500%. As a result, not only does the CPU have to keep calculating all the moving grass blades, your GPU has to draw all of them too.

Just replace the descr_grass.txt that came with the RTR mod with the original RTW descr_grass.txt file and your FPS will vastly increase.