Is there a way to set the processor speed of a Virtual Machine? I'm currently using VirtualBox on a Mac OS X Host, but other options would also be interesting.
I'm not aware of a Hypervisor where you can say you want a x GHz CPU, but CPU quotas might be what could help achieve your goal. A quick Google search yielded me http://cpulimit.sourceforge.net/ which has planned Mac OS support, though I couldn't find a downloadable version for Mac OS, so I'm not sure it will work. With that tool you can limit the maximum CPU utilization of a process to a specified value, thus effectively limiting the CPU speed (say you have a 2 GHz CPU and you want a 1,6GHz CPU in your VM a limit of 80% will do roughly what you want).