when your ac power goes out your laptop is configured to reduce the amount of power used by the laptop to increase the battery life and reduce discharge rate
in the case of gaming the laptop is using a lot of power because all the components in the laptop are running in their maximum ability,
so when the ac power is swithced of the laptop will get much less power from the batteries thereby the laptop will not be able to work in its max potential.
so this is nothing to worry abt,this also happens in my laptop also.