OK, didn't know that it could cause this kind of problem. Thanks for your response.
Currently we don't have plans to add this because it's too big server load.
You can use custom menu plugins like Ubermenu if you don't care about load
on server and speed
It would be awesome to get a custom menu for mobile. Now it takes the primary menu but I would like to add some extra menus only for mobile.
Is there a chance to get this option in a future update ?