I wish I could tell you exactly why a report might be so much slower in report manager vs. BIDS, especially when the report is referencing a parameterized stored proc.

Nonetheless, based on my own experiences, when a report does run slow in report manager vs BIDS, I usually discover a parameter sniffing problem.

So, here are a couple of links to articles about overcoming parameter sniffing:

http://elegantcode.com/2008/05/17/sql-parameter-sniffing-and-what-to-do-about-it/[/url]

http://www.sqlservercentral.com/blogs/practicalsqldba/2012/06/25/sql-server-parameter-sniffing/[/url]

--Pete