Do you mean you would like to do On-Demand scans of the server instead of having the real-time scanner running?
That is possible, though not ideal.
Wouldn't you want to look into why the system is bogged down when real-time scanning is On?
The McAfee Profiler tool can help you understand the activity, and from there you could look into making configuration tweaks.
There are other common causes of performance overhead too. It would probably be best to work with Support to hammer out the details.