Check the ExecutionLog table in Reporting Services to see how much of the time the report took to execute was query time and how much was rendering time.  Also check the settings for SSRS and SSMS.  Settings can vary depending on provider and login, and differences in default language and ANSI_NULL settings, for example, can change the way that the query runs?  Does the SSRS query run over a slower network link?  Compare the execution plans for the SSMS and SSRS queries - do they look the same?  Is the query in question the only query that the report runs?

John