Hi Swadesh
Please let us know certain things :
a) When are you observing system performance degrading ? Is it while running any specific report or transaction ?
b) Is it ECC or BW system ? which SAP version
c) Was there any recent change in the system or is it a new system
What are the response times you are observing in ST03 , how is the memory consumption looking in ST02.
From a DB level - please check the below SAP notes .
618868 - FAQ: Oracle performance
793113 - FAQ: Oracle I/O configuration
Let us know for specific observations ..
Thanks
Rishi