Then you're emulating everything that runs in the VM, as opposed to using an ARM vm and only emulating the x86 program you want to run. This makes things a lot slower.

That’s a good point… but does Rosetta in a Linux VM somehow use the native MacOS hypervisor?

I would like to see a benchmark of these various methods…

I don't know how Rosetta 2 for Linux is implemented in Virtualization.Framework - but it exposes it via a mountpoint on the (ARM) Linux VM. The binfmt executable that is exposed essentially does what Rosetta 2 does but for a Linux binary.

I think the binfmt executable can be used outside of Virtualization.Framework and can even be used in an ARM VM in say Asahi but people don't because it's not easy to do but also honour system as it's not licensed for use outside of MacOS.

You can't do Rosetta 2 for Linux thru qemu as qemu doesn't use Virtualization.Framework. UTM does support virtualisation via VF.