Collapsible items in menu administration
The Big Menu module seems to do the job. Here is a quote about it (from the module's project page):

An alternative to core menu management - adds scalability to the admin menu management screen, by loading subtrees via AJAX (not related to megamenu &amp; similar display modules - this is an admin-only content management utility).
When dealing with a site menu that contains thousands of links, the Drupal Menu UI admin interface becomes so unwieldy as to be impossible to use. When the drag &amp; drop functions for tree management are running, a page can take minutes to render.
This module attempts to replace the all-in-one global tree view with a similar interface, but sub-trees are &quot;collapsed&quot; below a certain level, and can be expanded as needed.