Hey, in this scenario, it is hard to improve performance, So I suggest to check whether the memory of the server is sufficient.
http://forums.asp.net/p/1442183/3269188.aspx
http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/4efcd028-f11e-4550-a8fb-f29463175f57
http://blogs.msdn.com/brianhartman/archive/2008/12/05/sql-server-2008-and-the-reportviewer-controls.aspx
http://blogs.msdn.com/selvar/archive/2008/07/18/getting-system-outofmemoryexception-when-using-reportviewer-contol-in-local-mode.aspx
local+mode+report+performance+reporting+services
http://www.eggheadcafe.com/software/aspnet/31460528/aspnet-web-app-very-slow.aspx
