Booting a VM should be "as complex" as fork. And the memory image of the booted OS should be identical on each load before the end user specific data is loaded.

"Prefork" these VMs by spraying that image over physical memory in advance, jump to them on a request. memcopy a new VM over them on exit.

VM spawn rate should be the memcopy rate, if the MMU is powerful enough, should only be the amount of memory mutated during a request.