Massive fps drops in foliage window.

Having to many foliage even if it has 2 triangles, added to work with for your terrain it drops the fps a lot. Tested on a GTX 980 video card.
I hope Epic can look into this. Fore the moment working with list is best option.

You can see here, I have nothing in the scene, and used the most low meshes to work with:

At first I thought this was an issue related to transparency, but it does look like a different problem all together.