This article explains how you can change the resource usage of your Imunify scans.
1) Log into WHM and navigate to Imunify. (WHM >> Plugins >> ImunifyAV)
2) Select the gear icon to the top right.
3) Change the settings as needed and save at the bottom.
4) You can also modify the time at which Imunify backups run from the same interface.
For more information, please see the documentation here.