The short version is that kernels support guest/host relationships natively so that guests can pass operations directly to the host without having to go through

The short version is that kernels support guest/host relationships natively so that guests can pass operations directly to the host without having to go through an additional system call. Everywhere you do this is attack surface where an attacker in the guest can communicate with privileged facilities, so you want to minimize this where you can. There's usually overhead in the places where the communication requires an additional hop. If you want your host filesystem isolated you're going to need a translation layer and it will be slower. If you're willing to open up your host OS's filesystem, you can basically get ~0 overhead.