Two other good articles on decreasing Pi boot times are:

- https://www.furkantokac.com/rpi3-fast-boot-less-than-2-secon...

- http://himeshp.blogspot.com/2018/08/fast-boot-with-raspberry...

I used these two to make a digital photo frame with a Pi that boots very quickly to a browser in kiosk mode. If you have very minimal requirements, you can get some very impressive boot times.