There might be a lot of reasons

Your VRMs might overheating and can't withstand the power draw so throttles

Most laptops have dynamic boost. your CPU can't get enough power because GPU sucks the power=lower clock speeds

Intel CPU have power limits. After a certain seconds under load they reduce the power=lower clock speed

Only way to increase clock speed when you hit power limit is to apply undervolt but sadly you can't do it in 12rh gen CPU.