You are using Exchange Synchronization with Exchange Server 2010. The user mailbox has "audit" turned and you experience the mailbox collection to be very slow and the following error repeated many times for each mailbox: "Microsoft.Exchange.WebServices.Data.ServiceResponseException: Access is denied."
In 3.9 we are missing the exception for retries on ErrorAccessDenied.
Add following setting to the ESS config:
<add key="ErrorCodesToSkipWhenGettingFolderChanges" value="ErrorInvalidClientAccessTokenRequest;ErrorInvalidOperation;ErrorAccessDenied" />