In a Microsoft Exchange Server 2010 environment, it takes a long time to expand a distribution list by using Exchange Web Services (EWS). For example, it takes about 20 seconds to expand a distribution list by using EWS. However, you can use Outlook Web App (OWA) to expand the same distribution list instantly.
The issue occurs because Exchange Server 2010 uses the wrong ADSession query.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.