Found this interesting: I got a vmware and a Virtualbox guest Windows XP, both same RAM, both expandable HDDs, run from same host SSD with quad core, vmware got 2 CPUs and Virtualbox only 1 CPU assigned. Guest OS config is almost the same (fresh installs) except the Virtualbox has a Comodo firewall and therefore (theoretically) a speed disadvantage. My host: Kubuntu 11.10 64 bit.
Vmware:
Boot 45 sec, shutdown 19 sec
Virtualbox:
Boot 27 sec, shutdown 11 sec
Difference: over 50%!
I find that interesting...