I’ve followed the Unreal Docs in this Tutorial: Grass Quick Start in Unreal Engine | Unreal Engine 5.1 Documentation
to introduce myself with Landscapes, Grass and foliage. But now, the frame rate is really low (about 30 when I look at the bottom at around 20 when I look to the horizont) and the grass is very dark, when I look to the horizont.
Do you have any idea how to fix that?
It looks, to me, like the darkness issue is because of eye adaption / auto exposure.
You can find settings for that on post process volumes that you can put in your scene:
More info here:
The skydome is, in real terms, quite a lot brighter than everything else in the scene. So when you look at the sky, everything else will go dark as your camera/eyes adjust to the sky.
You can tweak the values for that effect, or turn it off depending on what suits you better.
As for the frame rate, hard to say without knowing the hardware!
But a great thing to get familiar with is the GPU profiler in UE4:
You may just need to start thinning out the foliage, or even simplifying the materials and meshes might help, but the profiler will hopefully point you in the right direction, or at least help you measure the effects of trial-and-error reduction better
