How to optimize custom grass?

Well, first, use the quad instead of the plane. It uses like 1/64th as many polys.

But there are, in fact, dozens of ways... however, each have their benefits and drawbacks. I'll list a few I can t$$anonymous$$nk of off the top of my head below, but I'm sure there are more comprehensive lists/guides to be found elsewhere online:

1. Static Batc$$anonymous$$ng - basically cram as many grass quads as you can into a single mesh, but that eliminates billboarding

2. Point Extrusion - define a mesh of points, then use the geometry shader to convert into billboarded quads

3. Height mapping - similar to #2, but use one repeating flat mesh 'tile' and pass a heightmap to make sure the same mesh can fit itself to any terrain.

Keep in mind, you're also not limited to one method. You can switch methods per LOD to make more expansive grass run well.