Awful nanite performance with procedural foliage

Hi everyone. I was playing with procedural foliage and nanite lately and, to my surprise, I found that performance is HORRIBLE. I have an open world (16km2) which I procedurally populated with different foliage (trees, rocks, grass, bushes etc.). Then I switched meshes to Nanite and FPS dropped from 60+ to 21 with no other settings touched.
Is there any optimization I need to make to foliage after I switch to nanite? I didn’t find anything different from what I’ve already done.

possibly too much strain on virtual shadowmaps. simplest way to check it to enable/disable hardware acceleration on shadows