Yesterday, I decided to install Yosemite public beta 6 (build 14A388b) and I have stumbled on a serious bug which probably impacts everyone who uses virtual machines(VMs) in any way. A VMWare Fusion (7) VM is nearly unusably slow, while the vmware-vmx process eats up every CPU cycle on every CPU core assigned to the VM. This causes even an idle VM to become unresponsive. I've converted my VM (running Windows XP with some old software I regularly use) to a Virtualbox VM and, while it works slightly better, exactly the same thing happens: I have one CPU core assigned to the VM and the VirtualBoxVM process constantly uses up all available CPU cycles on that core, constantly running at 100% usage, even when the VM itself is completely idle. My first thought was a bug in either VMWare Fusion and VirtualBox, but more people on the VMWare support forum also mentioned exactly this problem. It would appear there's something wrong with Yosemite. Any thoughts?