If you do it right, you should be able to get a large landscape with a fair amount of foliage to run ‘reasonably’… If your default empty level is 120 fps, after making this level it could easily be down to, say, 60 fps or lower. It really depends on machine spec, but that’s a rough guide.
If it’s a large landscape, don’t use the foliage tool. Why bother when you can do it with a landscape grass type. Either way, you grass and rocks etc will be instanced.
What framerate are you getting, what’s your machine spec?