This is so true. Back in the day when I was involve in embedded Linux development, the quickest bootup time was about 40secs. This was booting a v2.6 kernel in a minimum configuration on an ARM7 system over SPI NAND FLASH. Hopefully, the bootup time is in the subseconds by now. Are we getting to these speed yet?
You can do that[0] with minimal system. Actually, it's not that difficult to get booting time down to few seconds on most embedded systems. But in real world boot time highly depends on modules and drivers that must be loaded to make system up and running.
I could possibly get it lower but it meets spec and any additional shaving off would probably require plenty of work while sacrificing debug-ability etc. so for now I'm fine where it is :)
The older NAND file systems were very slow. I've had a similar case where repartitioning the huge NAND into a smaller partition and only using a small partition reduced the boot time to 10 seconds without any other change. Modern UBIFS systems apparently don't have this problem.
As for subsecond boot times: no chance with Linux.