It appears to me that you’re just building everything and thinking Unreal will run everything perfectly. With foliage, the calculations can get extremely heavy especially if its dynamic. And the speed of your own machine will bottleneck what you can process at any one time.
To start, you might want to fix those lightmap issues. It’s telling you in the error how to fix them. How large are your textures?
Also fix the overlapping UVs. Use 1% or 2% spacing when laying them out.
As to performance how are you optimizing your scene? Are you using LODs for the trees? Do you have them distance occluded or are you using occlusion volumes? Those are a lot of leaves and they require a lot of calculations. Are they individual leaves or are you using sheets?
You might want to check these links for information on how to optimize your scenes.