Backport (7.9) - UI becomes unresponsive when trying to move a menu item in a big menu

Dragging an item in a big menu quickly leads to 100% CPU usage and very slow UI feedback. The dragged item hangs somewhere and the UI becomes unresponsive for up to 10 seconds. Subsequent drag attempts often lead to selected text, which is very confusing.

It also seems that the menu only starts working properly after first waiting for several seconds (in case of large menus).

Test case: the documentation menu on cms.onehippo.org.