During migration from Exchange 2003 to 2007 we had the problem with Outlook 2007 client free busy response was slow , about 10 seconds to load .

To fix it .
Open registry editor on your Desktop(where outlook is installed)

Navigate to the following path : HKEY_CURRENT_USERSoftwareMicrosoftOffice12.0OutlookOptionsCalendar
Create new DWORD value named UseLegacyFB and set value to 1

Note:
0 or not set will use default outlook settings
1 will use Public Folder based Free/Busy information

Read KB958934 to get more information.

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