I have reverse camera connected to SBC with Allwinner H6. It easily saves around 5 seconds compared to Pi (their closed bootloader takes ages to even start loading kernel). I considered early booting the system when opening the door, but really it is fast enough already.
Not too terribly sure, but NXP puts out power consumption numbers and they get pretty low in sleep modes with self refreshed DDR and whatnot. I think it's just something the market hasn't demanded enough. I'm not really a SBC person, usually just uC, where it's trivial to get microamps of sleep current. I'm sure there's an SBC out there that allows for it.
I have reverse camera connected to SBC with Allwinner H6. It easily saves around 5 seconds compared to Pi (their closed bootloader takes ages to even start loading kernel). I considered early booting the system when opening the door, but really it is fast enough already.