I'd be curious how long such a kernel takes to boot. There could be useful applications in container hosting.

Measured in milliseconds typically.

There was an example running on erlang-on-xen (later renamed ling) which spawned a vm per request. When not hammered by traffic from aggregator sites, it could respond in a few milliseconds.

No, it was not Jitsu. Jitsu is based on MiragOS/OCaml, Erlang-on-Xen was also a Xen-based unikernel like MirageOS but was (as you can guess) Erlang specific. I think you can now find it at https://github.com/cloudozer/ling .