In the MMC, by default, the Maximum number of items displayed per folder is set to 2000. This can be adjusted by selecting View | Filter Options.
In some environments, performance issues or errors have been noted when performing Find requests or attempting to display OU's in excess of 10,000 objects. Currently, there is no known workaround for these issues, other than performing an Advanced Find which returns less than 10,000 objects. Similar issues are usually noted in the native Microsoft utilities in these cases.
Some environments may encounter similar issues with Managed Units and Dynamic Groups queries which result in memberships in excess of 10,000 objects.
Performance can be impacted in the MMC when the Advanced Details Pane is enabled while viewing a large number of objects. This can be alleviated by deselecting View | Advanced Details Pane
The largest impact to performance in extremely large environments is the connection to the Domain controller designated as the DirSync Server. In extremely large and complicated environments, it is strongly suggest to configure a dedicated DirSync server for use with ActiveRoles Server, with a low-latency connection in the same datacenter or on the same Virtual Machine host. For more information regarding configuring a DirSync server please see Changing DirSync Server and Operational Domain Controller from within ARS
For more information regarding monitoring and possibly improving performance in Active Directory, please see Monitoring Performance in Active Directory or contact Microsoft.