Some of that can be attributed to faster IO.

Something else to consider: chromebook on arm boots significantly faster than dito intel. Yes, nowadays Mediateks latest cpus wipe the floor with intel N-whatever, but it has been like this since the early days when the Arm version was relatively underpowered.

Why? I have no idea.

My guess would be that ARM Chromebooks might run substantially more cut-down firmware? While intel might need a more full-fat EFI stack? But I haven't used either and am just speculating.