In a 2008 Sun Microsystems paper, the performance impact of virtualisation vs dedicated hardware was measured at 4.9% (1)

Reduce the number of vCPUs in your virtual machines to the minimum, as larger numbers of vCPUs will incur more scheduling overhead. For example, if running on a four vCPU virtual machine with only 30% CPU utilization, reduce the number of vCPUs to two and expect to run at around 55 to 60% CPU utilization. (2)