Hello,
I've been having this behavior happen from time to time in me VMs. It is more prone to happen in a Win7 VM, than in a Win 8.1, but it happens in both. I feel that it has something to do with not-using the VM for a while: say, spending some time on the Host OS, or in another VM. And when going back to the "problematic" VM then you can see a high cpu usage (using ProcessExplorer), all in Red (which means it's kernel time) and the performance of everything in the VM goes down the toilet. Even moving windows is sluggish, and so on. It sometimes "corrects" itself but usually I have to turn off the VM and start it again to be able to use it. Specially with the Win7 VM.
I haven't been able to trace this to something special, so... anybody here experiences the same?
I usually have my VM's in FullScreen mode. I have a MBPr with 2 external displays connected (and the lid closed, so I only have 2 displays). I do use some VMs on Windowed mode, and usually have 2 VMs at least opened: one in windowed mode where I handle Mail, IM, and so on, which is Win 8.1, and another one where I'm working, in FullScreen mode, using both monitors. Now that I think about this, I don't remember having this issue in the windowed VMs, only on VMs that use Full Screen. The MBPr has plenty of RAM (16Gb) and space available (around 300Gb most of the time).
Any ideas?