Hello,
SEP 12.1 minimizes the impact of a scan to the end user by improving on the existing scan throttling settings. Scan settings are configured via SEPM AV policies.
Scan Tuning initiates "Scan on Idle." This new feature works with Scheduled Scans, On Demand, and Active Scans. (The Right-click scan in Windows Explorer runs with "Best Scan Performance")
Secondly, check these Articles:
About the default Virus and Spyware Protection policy scan settings
http://www.symantec.com/docs/HOWTO55223
Adjusting scans to improve computer performance
http://www.symantec.com/docs/HOWTO80964
Adjusting scans to increase protection on your client computers
http://www.symantec.com/docs/HOWTO81016
Customizing administrator-defined scans for the clients that run on Windows computers
http://www.symantec.com/docs/HOWTO81013
Hope that helps!!