I wonder how much faster dos would boot, especially with floppy seek times...

Instantly.

If you run a VM on a CPU like this, using a baremetal hypervisor, you can get very close to "everything in cache".

You can get close with a VM, but there's overhead in device emulation that slows things down.

Consider a VM where that kind of stuff has been removed, like the firecracker hypervisor used for AWS Lambda. You're talking milliseconds.