TYPO3 Core

aside to some small configuration data the status of the pagetree for a user is stored serialized in the field be_users.uc.
Having a big page tree and editors which have to access pages in the whole tree, the serialzed data of this users gets bigger and bigger as states of opened/closed folders are only added to the serialized data. as this data is accessed for each BE-action of the user, the backend responses more and more slower.

either split the data as the page tree must not be updated on every click,
or implement some routines to reduce the size of the stored data: implement a button for cleanup the pagetree status information or do an automated garbage-collection (remove information about subtrees for closed folders, but only if the user activates this behaviour)