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).

Basically no impact. At least for me. Some people seem to get terrible performance, but I don't know why. Their drivers? Or is it trying to run without Virtualization enabled in BIOS? AMD-V, VT-x, VT-d?

I don't consider it odd. It is the way a Type 1 hypervisor works. Windows puts in some effort to hide most of that from the user.