If Outlook is in Cached Exchange Mode, and you are also using ActiveSync to synchronize a Windows Mobile Device, when you synchronize online data with the offline cache, Cached Exchange Mode may abruptly slow down and take a long time to complete the synchronization.
Additionally, if you are also using Microsoft Exchange ActiveSync to synchronize with a Windows Mobile device, synchronization with the Exchange Server mailbox may take longer than expected.
This problem occurs if you configure Cached Exchange Mode to download headers before downloading full items during the synchronization process in Outlook. If Outlook only receives headers during the synchronization process, ActiveSync activates, overrides the synchronization settings in Outlook, and then downloads the headers and their corresponding full items one at a time.
Follow these steps to work around this problem:
On the File menu, point to Cached Exchange Mode.
Click Download Full Items.
You do not have to restart Outlook to use this new setting.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.