This may be completely wrong, but as I understand it when you enable hyper-v it sits above your Windows install, and runs your windows as a hyper-v VM. So, rather than running a userland vbox process that has OS overhead (especially around disk IO) it can run much closer to the metal.

If I remember correctly, Hyper-V is not strictly a type 1 hypervisor ("runs your windows as a hyper-v VM") but comes close. The VMs still run pretty close to the metal, but your host OS is mostly free from hypervisor interference.

Because it’s a well known fact that VirtualBox has been dodgy slow for years. Especially around I/O, it’s been the laggard.

VB has lagged in support for good hardware acceleration, it's been an issue for years.