Last time I ran it, Lakka (a Linux disto for Retroach emulation appliances) booted in 5s or less on an RPi2, to a usable graphical menu, complete with particle effects or whatever if you have that stuff enabled.
>>JPLeRo+Sa4
Nah, but QNX could maybe do it still :-)
1.44 MB floppy read times might prevent sub-5-second boots to a GUI, though. I can’t really remember how fast things like that booted from a floppy, anymore.