Virtual machine running faster than source build. Why?

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Hi all, a fairly general question. I've installed sage both on a virtual machine and from source. I would expct that the source build would be more powerful as a virtual machine can only use some but not all of the system's power. When implementing a primality testing algorith (3000 digit primes), the virtual machine is significantly faster and is so on other large computations. Can anyone give a reason for this? I'm running it on El capitan (mac), core i5 intel processor with 12GB RAM. Additionally, the amount of RAM being consumed is nowhere near the full amount available.