Vaguely related: there's Firecracker which boots in 125ms on x86 but that's as a VM, so it's an apples to oranges comparison. From what I recall Firecracker powers AWS Lambda so it's an interesting project in that respect too.
I know Google cloud VM's are using kexec for faster launches; cause we had some awful toolchain related bugs to fix there. Debugging wasn't very fun, at least on x86 this part of the kernel is called "purgatory" cause there is literally no runtime (not even the kernel's "runtime" is available, mid-kexec).
https://firecracker-microvm.github.io/