I’m guessing the reason you are not seeing proper shadows is your directional light is probably stationary instead of movable? There is also an option in the foliage tool for showing shadows, make sure that is on.
Have you done a GPU profile to see what is slowing it down?
with lots of meshes + shadows you are probably going to have a bad time in VR.
To make it look as if it is moving add the SimpleGrassWind node to your corns material.