could be an overheating problem or the cpu isn't getting enough power (Battery issue)
#cpu losing mhz while playing video games
molten breach
brittle raven
Battery itself could be the issue not the adapter used to charge the battery.
eager quest
But its a laptop right? Some laptops also use the battery as well as the power brick to give enough power to the system
molten breach
if that's the case the u probably need to use an app like QuickCPU because the laptop itself might be throttling the cpu, used it on my old computer with i5 3th gen and it gave me extra frames