It’s kinda hard to tell without any details, a lot of different stuff might contribute to that spike.
What exactly in your scene, what tool you’re using for instantiating the grass, how big is transparent part of meshes, material setup, static mesh setup
For example, if you have 13k of movable meshes containing 20 overlapping planes where 90% of pixels are transparent and all of them are shown on the screen then you will get that crazy rendering cost. Or something in material might cause this