The level of virtualisation, or the way resources are shared is also rather critical. Fully virtualised servers are not necessarily all that efficient either,

The level of virtualisation, or the way resources are shared is also rather critical. Fully virtualised servers are not necessarily all that efficient either, especially if they're configured to do simple resource dedication among the hosted systems/processes. Shared systems can wrap their hosted processes with very fine-grained utilisation monitoring and reallocation, as well.