Realtime-critical applications previously struggled in virtualised environments. For example, certain TTS engines running on Linux SMP VMs on VMware Server resulted in 'jittery' audio being produced; the same timing issues caused certain ASR engines running on similar versions of VMware to struggle to recognise voice input correctly.

That was only a couple of years ago, the situation seems much improved in the ESXi 4/5 realm (in conjunction with more recent Linux releases such as CentOS 5/6)...

http://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf