Hello,
Believe me , Kurt is correct.
Let me get things right, you want to improve performance as well as keep the scan Enabled...
We see that you are very much interested in having your scans more of Performance based and should use as much as less CPU usage.
Check these documents as below:
1) Enabling multithreaded scans
http://www.symantec.com/business/support/index?page=content&id=TECH101387&locale=en_US
2) Symantec Endpoint Protection scan tuning options
http://www.symantec.com/business/support/index?page=content&id=TECH105706&locale=en_US