One thing that Laubwerk has done, which is very nice, is the ability to swap out materials for faster rendering. They have a low, medium, high set that you can import for each tree/plant.
I find that the grass material setup from the Skatter library just takes too much time for fast renderings with lots of grass areas. That 2-sided material kills rendering times and the glossy reflection. Both might be unnecessary for rendering a lawn.
I would also consider a version of grasses made for 3.x version of VRay using the BRDF material type, especially for those planning to use GPU rendering.
Maybe the easiest fix is to just create a 'fast grass' family with not 2 sided or reflection.