Furs pretty much similar to aint effects on the rendering front so I probably wont help you out on your rendering problems.

If you mean it takes long to render then that just part of it, of you mean it slows your PC down in maya just lower the quality of the display for the paint effects, or add them to a sepperate layer and make it non visible.

Other than that if you want realistic grass your going to have to have long render times, thats part and parcel of creating something realisitc.