virtualbox.org

Hi,

I have performance issues with any versions of Windows I'm using. Ressources should not be an issue.

Macbook Pro 2012 non retina
i7 2.3GHz (3615QM)
2x MX100 256gb in RAID 0
16GB Ram DDR3

I gave my VMs 4-8 GB of RAM and up the CPU to 4 cores. The VMs are often not responsive under light workloads.

Fullscreen seamless mode is disabled.
Running Win 8.1 all current updates installed
or
Windows Server 2012 R2
Yosemite 10.10.2
Virtualbox 4.3.20 r96996
Guest additions installed

Did you try your VMs with just 2 virtual CPU cores?
Your host only has 4 cores, so when you give the VM all of them nothing is left for the host.

post the vbox.log (compressed) of your guest.

here it is

According to the log you posted you have 8 cores assigned to the guest. That will never work.

00:00:00.827730 NumCPUs <integer> = 0x0000000000000008 (8)

Try assigning 2 cores and 2 GB RAM for the guest and see how that runs.