In truth it's not; however if the software has to do extra things like, for instance, translate io calls to a virtualbox disk format that hyperv cannot natively support or do an extra memcpy on the video framebuffer to get their UI to work then there will be necessary performance impacts. How fast a guest os "feels" is mostly down to the performance of the virtualized devices and not necessarily the overhead of virtualization itself.

Yes, it is; even the official documentation mentions it (and recommends you disable Hyper-V) and it is FAQ #1 in the support website.

One of the reasons mentioned is that VirtualBox runs (some) emulated devices in kernel space but is not allowed to do when running with Hyper-V. The official API forces custom devices to strictly be user space, and only some basic hardcoded devices are emulated from kernel space.

The "secret sauce" of a desktop virtualizer is in part in the selection of devices it emulates, so this severely cripples VirtualBox.