Foliage optimization in 4.9

We’re running into constant issues with the foliage in our level, which lighting fails to build on. As you can see below, it’s already so sparse that it’s almost useless, and the whole level is maybe ten times of the area in screenshot. Foliage tool shows there’s 60k instances of the grass mesh. The lightmap resolution on each is at its lowest - 4x4

Any ideas what we can do to optimize the performance? Is it possible to somehow enable dynamic lighting for folaige only, maybe through a sublevel? What are our options.

https://i.sli.mg/C8DNpS.jpg