Can you limit cpu usage before a backup begins?

I have Ghost 15.0.  Once a backup begins you can go to Progress and Performance and adjust the slider to control how fast the creating a restore point takes which limits the amount of cpu time.  I need to do this in advance as I have my backups scheduled for late at night but also am using three of my four cpus to do scientific calculations.  Unfortunately the default setting seems to be use as many of the cpus as possible.  Is there a way to change this?