When you synchronize between Microsoft Lync Server 2010 Group Chat server and Active Directory Domain Services (AD DS), a principal may be synchronized multiple times. Therefore, the synchronization process takes an extended time to be completed. Additionally, the channel service log indicates that the same principal is refreshed multiple times.
For example, during the AD Sync operation, when a single user is part of many groups, it can take up to 10 minutes to synchronize. If the same user is synchronized 6 times, it adds an hour to the total synchronization time.