When outlook users are complaining about slow performance and your cas/hub server(s) are in another AD site it may be worth to set a registry key to let the user’s pc use the nearest Global Catalog.
IMPORTANT: This setting can cause issues in Exchange 2010 regarding Address Book Policies! Only use this setting when experiencing latency issues!
The registry key to be added will be located in HKEY_Current_UsersSoftware\MicrosoftExchange\Exchange Provider On the Edit menu, click Add Value, and then add the following registry value: Closest GC, Data type: REG_DWORD, Value data: 1
For an individual user this can be set manually but if this has to be set for a whole location, it gets a bit more difficult. Here is where a group policy comes in. Open the group policy management console and create a GPO in the appropriate OU. Then under User Configuration go to Preferences (only available on Windows 2008 and higher servers), Windows Settings . Right click the Registry setting and choose New, Registry Item .
In the new window choose the following settings:
Click OK and save the GPO. Once the user logs in again, this new registry setting will be entered.