[Steve]

Let's ignore the boot process and assume we're resuming from hibernation.

How many devices are there on the system? Not just peripherals, but devices... timers, DMAs, interfaces etc.

Now the idea of hibernation is that you dump your whole memory onto disk, then load it back up and pretend like nothing every happened. Except it's a lot more complicated than that.

Lots of these devices, that you've just woken up from 'cold' again, only expose certain parts of their registers and internal workings to the system through memory mapping. They may need some initialisation procedure to bring them back to the pre-hibernation state. You can't just write the registers back and hope for the best. So every driver needs to know what to do when the OS says "Oh hi, I'm coming out of hibernation, sort your device out plz".

And that's just one example of what complicates and slows down coming out of hibernation.