Well..The Scan performance impact should have been reduced by the latest version.
Would suggest making neccesary Exceptions and Scheduling the Scan outside/off office hours.

if you find any files here. try deleting it and check if CPU goes down
when it runs the scan, it will scan the quarantine and will take high cpu

C:\Documents and Settings\All Users\Application Data\Symantec\Symantec AntiVirus Corporate Edition\7.5\Quarantine folder

The scans are scheduled to run at night. If the computer is not on when it is scheduled to run the scan then SEP will scan when the computer when it is  turned back on. My laptop was not on during the schedule scan so when I got back to the office and powered it on. It ran the scan. The list of Exceptions has not changed nor do I feel that adding more will effect the CPU utilization. I am just noticing that the new client does not scale back as fast and as the old.

I have just updated to MR5 on my site and have encountered that by doing this it improves the systems performance. Also the directory specify by Rafeeq was also caching files related to SEP's scanning. I found that several users were reporting low disk space and performance hits. After upgrading the files not longer were longer being cached and also removed the existing files. I went from version 11.0.2 to 11.0.5. I think this also might have an impact on your Real time scan. Also, consider your machine resources and determine if it is based on processing power or lack of memory. Scale your dedication to system performance to give the system more power to run applications if you are running SEP on older clients machines. The scan will run longer but it will allow for less impact on the client system.