thanks for the followup, glad you could ask others about this 
so I tried what you suggested, and I see that it helps a bit but it doesn’t quite do it. and it makes me more confused.
I tried unchecking ‘Enable Collision’ for every Element of every LOD, including LOD0, and I was still colliding against the collision primitive of it (the capsule). which I find extremely odd because in theory it should be always using only that - no per-poly collision is enabled so why should it make such a difference, any difference at all?
so now I have my scene with one tree type scattered around the land (an area of 3x4 km), for a total of 47k trees. I set all the Engine Scalability Settings to the lowest, and then I spawn my character and not move the camera. and I get this:
- With all collision enabled in the mesh LOD elements, and foliage set to NoCollision: max 78 FPS
- With all collision disabled in the mesh LOD elements, and foliage set to NoCollision: max 78 FPS
- With all collision enabled in the mesh LOD elements, and foliage set to BlockAllDynamic: max 16 FPS
- With all collision disabled in the mesh LOD elements, and foliage set to BlockAllDynamic: max 32 FPS
number 1) and number 3) is what I reported in the original post btw.
number 4) is what you suggest. so your suggestion seems to help, a bit. I can now collide against the trees and my framerate is “only” butchered to a bit less than half. definately not a way to solve the problem