The crowd isn't the problem. I gain only a frame or two with them out. I think the biggest problem is the trees and grass. For one the grass needs a much more aggressive LOD system. Here is a paste from a conversation I had with Mitja about Okayama and his grass.
"Number per section I don't really care about. It's the length of the section that concerns me.
I do something like this over the grass. In this case each section is about 500' long.
View attachment 212783
I then cut out sections of the grass according to the pattern.
View attachment 212784
The key thing is that the origin or pivot point must be at the center of all the new separate grass objects.
For Bridgehampton I have 5 layers of grass all cut into the same sized pieces. Here you can see all 5 layers selected for one section. Notice how the origin points are all within close proximity of each other.
View attachment 212785
Some base LOD #s for 5 layers is:
KSLAYER1 = LOD_OUT=350
KSLAYER2 = LOD_OUT=300
KSLAYER3 = LOD_OUT=260
KSLAYER4 = LOD_OUT=160
KSLAYER5 = LOD_OUT=80
For a 3 layers system like Riverside and some Kunos tracks:
KSLAYER3 = LOD_OUT=350
KSLAYER4 = LOD_OUT=170
KSLAYER5 = LOD_OUT=80
A 5 layer system has less noticeable pop-in or 'crawl' than a 3 layer system.
Here is a final overall shot of Bridgehampton grass. You can see the origin points of all the objects.
View attachment 212786
As for LOD#s for other objects I don't really bother with them much. Only with the grass have I found the most FPS gain. In either case if I do set LOD #s for an object I just look for blatant pop-in and adjust the numbers to avoid it."
As for the trees I don't really know what is going on. I have a feeling they are just too dense and the amount of shadows the sim is trying to render is too much. It sounds like you have a decent rig so I can see you not having issues. They thing to pay attention to is when you might have a 5 FPS swing on your rig it might result in a 10+ FPS swing on a lower end machine. Going into the first tree section I see 22+ FPS drop between the first bridge and the end of the woods section which is huge and that is without the grass and 3d people being on. Also a good practice is anything under the trees should not be trying to make shadows. This would be the bushes and small trees that are under there.