Amazing, great job! Only a few things: better forest immersion (add another row of trees? but maybe it's not like in real life so not very sure about this one), better mud (i think it's mud but perhaps it's gravel?) textures, and a warmer illumination. How ever, the track looks beautiful!
if you want to take a shot to area where is FPS problem and send to me so i can focus to fix this issue :)