Hello Devesh,
My first recommendation is to ensure the issue is exclusive related to workbooks.
Try at transaction RSRT2, execute the queries of this workbook and check if the performance issue occurs there as well.
If it is Workbook exclusive, please read this Wiki page:
> Optimizing BEx Analyzer 7x Performance - SAP NetWeaver Business Warehouse - SCN Wiki
If the issue occurs on RSRT2, it is a little more complicated.
Check this page. This will help you to figure out what is taking too much time to execute
I hope it helps. And I hope the issue is workbook exclusive. 
Kind Regards,
Edward John