1. Set Terrain to Very High or Ultra on a rig with an i7 2600k/580 GTX/16Gb DDR3 1333Mhz RAM configuration equivalent or lower (posibly on better rigs but I can't say).
2.Go to grassy area and note the low performance.
3. Go to area with minimal grass within draw distance but a long distance view of Terrain Geometry. Note the improved performance.