What is happening is that during charging, the tablet is actually on. If it was off when you plugged it in, it will turn on and go to a state where everything is booted except the desktop. When you press the power button, the desktop gets loaded really quickly and *presto* quick boot time.

If you press the off button while it is plugged in, it will not actually turn off but rather just shutdown the desktop and go into charging mode.

The trick here is to never turn off your tablet. Just plug it in when it needs juice. With the screen off (cover on), no media player running and no wifi connection, the battery will last several days in this state.

The only reason to to a full reboot is to unstick some app that went belly-up and you have to do this while the tablet is unplugged otherwise it won't fully poweroff.