How system utilization works
The on-demand scanner uses the Windows Set Priority setting for the scan process and threads priority. This lets the operating system set the amount of CPU time that the on-demand scanner receives at any point in the scan process. The system utilization setting in the On-Demand Scan Properties maps to the Windows Set Priority control.
Setting the system utilization for the scan to low provides improved performance for other running applications. The low setting is useful for systems with end user activity in progress. Conversely, by setting the system utilization to normal the scan completes faster. The normal setting is useful for systems that have large volumes and very little end user activity.