When you try to open email messages by using the light version of Outlook Web App in a Microsoft Exchange Server 2010 environment, you experience performance issues. For example, it takes 5 seconds to open the email message. Or, when you click an email message several times before you open it, a time-out error occurs.
These performance issues do not occur when you open the same email messages by using the premium version of Outlook Web App.
When the issue occurs, you may experience high CPU usage on the domain controllers that are used by the Exchange servers in the organization. This high CPU usage does not occur on other domain controllers that are not used by the Exchange servers
The issue occurs because Lightweight Directory Access Protocol (LDAP) queries are not optimal when Exchange servers and Outlook Web App light clients perform mailbox lookup operations.
To resolve this issue, install the following update rollup:
2706690 Description of Update Rollup 4 for Exchange Server 2010 Service Pack 2
Note The fix optimizes the LDAP query. For example, before the fix, it takes 15 seconds to execute the old LDAP query. After the fix, it takes 38 milliseconds to execute the new LDAP query.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.