Take a look at McAfee Profiler ; should help in dialing in OAS/ODS scanning:
McAfee Profiler captures top processes and files that are accessed by the VirusScan Enterprise (VSE) On-Access Scanner (OAS). Based on the data collected, an administrator can choose files or processes to exclude from scanning to lessen the impact on the system.
It's possible, yet too many variables to consider. McAfee Profiler may help dial in what is actually being scanned AND more importantly - what shouldn't be scanned (which is all govern by your environment culture and scanning/regulatory policies).
Every environment is different yet here's a good starting point for AV exclusions: McAfee Corporate KB - Consolidated list of Endpoint Security/VirusScan Enterprise exclusion articles...
Login session may be scanned and then stop; to my knowledge it shouldn't continue to scan. With that said, this is all dependent on the login user isn't performing an action to trigger an "OAS" to happen (such as: install, server re-config, software monitoring ... and so on).
Only increase the consumption of resources during the scan, should return to normal after finishing the scan, you should verify that the scan time is once a week and that it is an hour, better if it is at lunch time.