Hi,
the reason why booting from cold is quicker than a reboot is that when you turn off the pc Win 8 enters a type of hybrid sleep. All services etc are stopped but not switched off so when the system is booted up again it basically starts where it left off.
Rebooting on the other hand means that everything gets turned off and then restarted again which is why it takes longer.
This site has more info:
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