Unfortunately I have no number to give you. I did a search on the subject a while ago and all that I could find is numerous post on multiple forums with people

Unfortunately I have no number to give you. I did a search on the subject a while ago and all that I could find is numerous post on multiple forums with people indicating slowness, lower fps in games, shuttering. Weirdly, this is something that could be tested pretty easily with a benchmark. In my understanding on how a Hypervisor works, the only thing that I'm almost confident to say is that there is an impact but it's probably not noticeable or highly related to your workload (maybe on I/O?). People will probably not care about that impact in fact (considering that Hyper-V is enabled by default and there are multiple security features of Windows now relying on Hyper-V).