I think some of these super-fast boot times are with Apple hardware only and no USB/TB devices connected. The more "junk" we fill our macOS with, the more devices we connect, in particular monitors and hubs, the slower the boot becomes.
My Mac Studio (specs in sig) takes;
- 7 seconds from power on to login screen - it then takes 3-4 seconds to recognize the keyboard and me to type my password.
- 8 seconds to get to 60%, at which point my setup has an issue (not sorted in Beta 5), and it "hangs" for 70 seconds.
- 5 seconds from 60% to desktop.
- 2 seconds for a few apps to load in the background.
So in total (not counting time at login, and my 70-second issue), from power-on to desktop, 22 seconds.
I am a natural skeptic, so I do not believe in a sub-10-second boot. Sorry!
I don't know why others are taking longer to boot. But I have to admit I used a stopwatch today and it took 10sec to desktop. So, it took a bit longer. Maybe because I added some new peripheral yesterday (printer, magic keyboard, different USB hub), will have to measure again tomorrow to see if it stays the same. It's strange, though, that the experience is so different for many users, e.g. some complain their Studio is very noisy. Well, again, mine is completely silent, even when I press my ear against the top I hear nothing. Even when working under heavy load like in Logic, PS or Final Cut. But I can always hear my (rather silent) external SSD. Anyway, never had a better Mac. I am very happy. 
