Do you remember any details that would let me search for it? Because that does sound cool, and even maybe useful; the thought has certainly crossed my mind that a router or VPN box doesn't really get a lot of use out of userspace... Although maybe it's worth keeping for control/configuration/debugging.
> To print some text or run a simple program, I belive DOS without a memory manager would be even faster.
Or just make your code boot directly. It's not hard to make a .efi, or use https://github.com/jart/cosmopolitan to make a binary that runs in many places including bare metal.
To reconfigure, the admin would simply reset it and start the system with "init=/something/else" as kernel parameter that booted to a normal userspace.
This is one of several major arguments made against unikernels in that famous Triton rant from a decade ago:
https://tritondatacenter.com/blog/unikernels-are-unfit-for-p...
Basically, even if your application _can_ run as the kernel, and it's desirable for it to run with kernel-level permissions, do you really want production to be a world without strace and iotop and the like?
Do do something actually useful, the program would have to access some data: network, disk, some sensors, etc. Network alone means scanning PCIe for the network card and configuring it, disk access needs controller also on PCIe, then scanning the ports for the drive, reading partition table, mounting the partition, etc.
All that takes a lot more than 1s. The speedup might not even be significant compared to a kernel optimized for that system (all modules built-in, nothing redundant), but full-featured, plus busybox or sysvinit alone.
It makes sense if you got some legacy piece of hardware that has extremely limited resources, both in terms of RAM and storage. Write your code in Go and you don't even need libc any more.