My hypervisor of choice is VirtualBox and it seems that the operating system it handles best as far as CPU performance is concerned as a guest OS is Windows XP. On the host system running Ubuntu 12.04 the SunSpider benchmark results in a score of around 420 ms with Firefox 16.0.2. Running the same test with the same browser in the virtualized Windows XP results in a score of 455 ms, i.e. only slightly slower.
On a Windows 7 guest OS, however, the score drops to 604 ms, quite a bit slower. And finally, an Ubuntu 12.04 guest also only reaches a score of 607 ms. In other words, while the Windows XP guest almost incurs no slowdown, Windows 7 and Ubuntu are 50% slower. That is very strange and I haven't figured out the reason for this yet.
Also, I can rule out that the SunSpider tests gives different results in different operating systems running natively. On another computer I have Windows Vista and Ubuntu 12.04 installed natively and the SunSpider results with Firefox 16.02 give almost exactly the same value.