Hi,
Any recent OS X requires 2 cores available to function well. So if your VM has been set to 3 cores on a 4 core host, it means that the CPU scheduler will regularly bump into collisions where it needs to schedule 5 cores, but only has 4 cores to work with. In that case one process will have to wait until sufficient cores are available to be scheduled.
Both your host OS as well as your VMs can end up waiting, the result is slow down.
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Wil