RPi 3 in 2s: https://www.furkantokac.com/rpi3-fast-boot-less-than-2-seconds/
RPi 3 3.5-4s: http://himeshp.blogspot.com/2018/08/fast-boot-with-raspberry-pi.html
RPi Zero 2 W in 3.5s: https://kittenlabs.de/blog/2024/09/01/extreme-pi-boot-optimization/
Discussion: https://news.ycombinator.com/item?id=41420597