Several users may have complain that Outlook 2007 freezes (or becomes extremely slow) until the synchronization between IMAP server and Outlook data file is complete.
Click “Tools” and then “Send/Receive Settings.” Click “Define Send/Receive Groups,” then “Edit” and tick the “Inbox” check box. Select the “Download complete item including attachments.” Change the “Spam” folder in the same way, but select the “Download headers only” option. Click “OK” to continue.
Adjust the time that your mail client queries the IMAP server for more messages. This is purely a preference setting and is not necessary to improve speed except that the more frequently you check your mail, the more up to date your in-box will be. Close the “Send/Receive Settings” menu to return to the main Outlook screen.
Click “File” and then choose “Work Offline” from the menu. This option will keep Outlook from querying the IMAP server every time you try to open a message and speed up load times.