This is a known fact that an antivirus program uses a lot of system resource while performing scan process. This makes the system perform in a sluggish manner. There are different ways through which user can avoid this issue. The first thing they should work on is PC optimization.
Scheduling your McAfee scan to the time when you are not using your PC can help you deal with the problem in a wise manner. Or otherwise, by customizing your scan you can exclude trusted files from scan process. This will minimize the resources used by McAfee, hence making your system perform better.