"I Would like to know the Best Achievable Rate of Configuration. is it device dependent and how to configure it while loading the bitstream"
-> This depends on which configuration interface you use and many other factors.
for example you can refer to page #21 "The High-Speed Priority Option" and page #56 "Determining the Maximum Configuration Clock Frequency" of following UG
http://www.xilinx.com/support/documentation/user_guides/ug360.pdf-Shreyas
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