Joey, fast boot is kind of like Hibernate mode on a computer. That means your device doesn't actually fully shut down when you turn it off (if fast boot is enabled). And the benefit of this is that the boot up time is shorter -- hence the name fast boot. It would probably take several more seconds for your phone to turn on if fast boot is disabled. It may be best to simply disable fast boot if you don't turn your phone on/off frequently and don't mind waiting those few extra seconds.