Is the light baked or is it real time? Have you tried moving a dynamic shadow casting light through the scene and seeing how that affects perfomance? Any stats on memory usage and latency would be nice… Could we have a shader complexity screenshot and a shot of the material would be wicked. How many triangles are being rendered in that frame. Is the grass moving or is it completely static?
is Post Processing quality at 0 here or do you have it at a normal value? How does it perform with a few hundred large shadow casting objects in place?