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I know its late, but wouldn't a "busy" CPU throttle the speed of the cards? If the CPU is busy mining, he can't handle the new mining jobs for the CPU, right?

Hi better late than never :)
The CPU tasks are set to the lowest priority, so literally any other task will always get CPU time before it. This works out as the computer using whatever spare % of the CPU is available.
Also if you like you can configure the number of CPU cores to use, but I never found a need to do that.