Speeding Up Outlook
When researching this, I saw that the guaranteed fix to this problem would be to download a program called "December 2010 Hotfix KB2459115", but there is also something else you should try first. It's pretty easy and usually effective. What you need to do is to remove the IMAP account, restart Outlook and then add the IMAP account again. As all the data of the IMAP account is located on the server, no data will be lost and Outlook will automatically begin re-caching the data from the server. Recreating the mail profile entirely could help too. Hope this helped!