First of all what the version you are running in your n/w?
How many clients are in n/w?
Version of that clients?
Configuration running on client?
Hope this below link is alos help in your issue
System requirements for the Symantec Endpoint Protection client software
http://www.symantec.com/business/support/index?page=content&id=HOWTO26664
Ensure Scan Tuning options are set for “Best Application Performance”:
http://service1.symantec.com/support/ent-security.nsf/docid/2008082509323748
Consider using multithreading during scheduled scans:
http://service1.symantec.com/support/ent-security.nsf/docid/2005062813030748
Consider utilizing the resumable scan feature:
http://service1.symantec.com/support/ent-security.nsf/docid/2005062806252148
One more attach link for the system requirement
http://www.symantec.com/business/support/index?page=content&id=TECH92098
If your system configuration is not come under recommended requirement then I can guide you that schedule the system scanning at non peak hour which help to reduce the slowness effect and no bussiness impact.
Hope you understand