high runs fine but extreme is lagging
I don't know if anyone with a "normal" pc nowadays can pull off extreme. It requires the very latest processor technology, so it's more about raw speed (e.g. Intel i7 proc running at very high speed). I know you have a quad core, but combined power does not work in this case. (Technically we're rendering all trees as meshes in extreme. In other settings, we're lodding the trees, so if they're too far off, you see billboards which don't require too many drawcalls.)
So it's not even about GPU, it's CPU
