I am working on the same question, created s imple scene with 5000 actors and rendered them as single actors, instance static mesh and hierarchical instanced static mesh.
In my tests the single actor version gives the same FPS as the HSIM. the simple instanced mesh version gives only half the framerate.
I am confused as to what this means as well
there is this tool on the marketplace (Instance Tool in Code Plugins - UE Marketplace) and the author stated in the description that since 4.22 UE has an “auto instance feature” which would explain the equality of single objets and HSIMS. Maybe creating HSIMs manually is no longer necessary.
I would also appreciate if somebody with knowledge about this could add an answer