Thanks for reply!
I have almost 38 000 actors in my scene, most of them are StaticMeshActor - 3D plants from Quixel Megascans. I didn’t use any foliage or landscape for it because I wanted to place them in a specific way.
It was working pretty well until some point.
Is it too much for Unreal or there could be a way to fix it?
Edit: Seems that merging meshes helped.