Veena,

Your observation about SSRS performance degradation in SharePoint Integrated Mode is not uncommon unfortunately. The specific solutions will depend on the versions of SSRS and SharePoint that you are running.

I have found the following references to be helpful:

Melissa Coates has a very thorough discussion of her issues in dealing with the same issue at:

http://www.sqlchick.com/entries/2011/1/25/whats-up-with-the-slow-ssrs-r2-rendering-in-sharepoint-2010.html

The SQL CAT team also wrote a white paper on this topic, "Reporting Services Performance in SharePoint Integrated Mode in SQL Server 2008 R2". While the original site has been removed, the paper was included in an eBook on BI and analytics that you can find at:

http://blogs.msdn.com/b/sqlcat/archive/2013/10/23/sqlcat-com-ebook-downloads.aspx

The specific paper begins around page 139.

Hope this helps,

Scott Thornburg