The issue is caused by the update kb2412171. Information from Microsoft is located here.
In order to resolve the issue go into control panel, Programs and features , and select “Installed Updates” from the blue menu on the right it will display all the updates. In the Office section locate the update numbered kb2412171, and uninstall it, the problem will go away. Make sure you close outlook before you do so.
While we are on the subject of outlook performance, it should be noted that the pst file where one’s emails, calendar items and contacts are stored tends to grow and fragment over time. An occasional cleanup of old mail items, archived or moved to another pst file, followed by compacting the pst can do some real good in keeping your outlook email client quick and responsive.