As you are using the RAW edition, it does as many parallel checks as it can.
The maximum of parallel checks are the number of your host objects.
But as you said if some hosts are blocking single cores you will run in a bottleneck of available cores for the workload.
That’s why you should pay attention to the execution times of the Check_MK service.
Normal server (Linux/Win) 0,5-2 seconds
Normal switch 0,5-5 seconds
Bigger switch/stack 5-20 seconds
Special agents like Netapp and HP MSA - depends mostly on the size of this device from 1 to 60 seconds all is possible 