It is quicker because Rosetta is a proprietary Apple technology which apparently works superb, but only supports userspace code. Emulating x86 with qemu does not work as fast, unfortunately. I'm not qualified enough to judge whether it's qemu shortcoming or it's just not possible to emulate entire VM as efficiently, but the practical result for today is that you want to use Rosetta.

Yes. If you run x86 Linux in a VM on an M1 you are emulating/translating every single instruction; with a native OS you are only translating the application.