All of the smartphones I've owned required to hold down the power button for several seconds (ranging from 3 to 8 in some cases) in order to startup from a shutdown state. I've wondered why was such a design decision made? Usability reasons or has it has something to do with the circuitry of the phone? I have never seen another electronic device starting up this way and im curious why they chose to do this on mobile phones.
From the electronics perspective, it seems to me if they wanted to implement the "Instant Power On", it was easy to do.
I think the reason for the delay is usability, considering this two aspects:
It makes it harder to unintentionally power it on, specially when it is on a pocket
Making the act of "Power on" symmetric to the "Power off" act, which makes the user need less knowledge of timings of the device responses.