To enable smp in virtuabox guest, one has to activate io apic. It happens that the host machine and also the guest machine become very slow. Current solution is to disable io apic y smp. Yes it's faster!

http://www.virtualbox.org/ticket/4392

To disable io apic in an already installed windows guest:

http://www.uberactive.com/2009/08/20/disable-io-apic-after-installing-windows-in-virtualbox/