Re: 3 seconds boot time?
NOT having SystemD would improve that. Devuan comes up really fast, booting into a GUI window manager (not gdm, I forget what it's called, it's lightweight). Evdn when I had it connecting wirelessly, it was still pretty fast. But ethernet is a bit faster I think. That box has an SSD on it.
Most of the boot time on my BSD boxen is due to all of the daemons I load. I never bother timing it and they all have spinny drives. I've never really minded, since they run for WEEKS (and months) without booting.
if BOOT TIME is all you're concerned about, a dedicated RTOS is probably going to be thee fastest. Whoopee.