It worked on the old version, in the front. And in the backend we mate some changes to make it work.
In the old version, there is a loop for each level, such child items is only loaded if the top parent item is clicked.
There was no topmenu for mobile back then, I guess thats why there was no need for all menuitems at the same time.
We are looking for a solution, where we only need to load the needed menu items, and not all 300K.
In the top level we only have 4 items.