Check your temperatures of your CPU and GPU. How hot does it get when gaming? It seems like your game makes your computer thermal throttle because games require processing power, which needs more voltage, and then more power, causing heat. When it gets too hot it will throttle to keep temps down.
You want CPU temps under 65C and GPU under 80C
If it's a CPU temperature problem, go in your case and press down on the four pins holding the cooler together and make sure it is thight, then make sure it's not clogged with dust iPad fan is spinning. I see this as most likely
If it's GPU temp, check that there's good airflow in the case and that the GPU cooler is not clogged and works.
Use speedfan, HWiNFo, or open hardware monitor, or...