Generally since you are using Koola’s assets, I can only say that he wasn’t worrying about performance at all when he was making those.
Other than that, I don’t know why but I just know UE4 is generally very bad with handling foliage reason being I do a lot of work in cryengine, most often 10 times more foliage there in a scene but no fps drop. I hope we will get to know why some day. 