Optimizing dense forest map.

I have a problem. I’m trying to optimize a map and I’m failing miserably in doing so. I’m making a dense forest with trees from Landscape content, custom made grass, and other low-poly meshes with LODs. I’ve set cooling. I have used one directional light and skylight. Sun is set to sunset.
Without previously mentioned light sources I have over 90fps, with it it drops to about 20. Ideas? Any help much appreciated!