In qlikview there creates a shered file where bookmarks are stored from users. so due to this file qmec become slow.
for ease of access you can take backup of shared file somewhere and then rename shared file . make the changes you want in QMec. after changes done you must copy old shared file.
and you can also delete bookmarks of user which are not in use.for this you can use sharefileviewer of power tool.
hope this helps