http://sqlcat.com/sqlcat/b/technicalnotes/archive/2009/01/14/reporting-services-performance-optimizations.aspx
This URL is an article written by members of the SSRS product team explaining some performance optimizations you can do. It's a good article and a great resource for this kind of issue.
Also, see here:
http://msdn.microsoft.com/en-us/library/bb522786.aspx