I've been using Oracle's VirtualBox for several months now. I'm really impressed with this product and it has helped me a lot to rapidly prototype some APEX concepts.
One thing that bothered me when I started using it was that no matter what my virtual machine was doing, VirtualBox was using a lot of the CPU on my laptop. This didn't matter if the virtual machine was idle or doing any processing. After some digging around I found this very helpful post on the Oracle forums: http://forums.oracle.com/forums/thread.jspa?messageID=9305831. As weird as it may sound the suggestion is to create and start a second, dummy, virtual machine. I was pleasantly surprised to see that it worked. The configuration that I used was: DOS as an OS. 4 MB of memory and a 4 MB hard drive. Disable all the extra stuff (network, audio, usb, etc).
Here are the screen shots of my CPU before and after I started the second, dummy, virtual machine:
After starting the second virtual machine the CPU spiked for a few seconds then dropped significantly. Hopefully this issue will be fixed in a future version of VirtualBox but until then this is a very simple work around.