I'd like you to check the temperatures when the game starts lagging. You're looking for CPU, GPU, HDD and RAM. (CPU, GPU and HDD has sensors, so you typically can get a program like hwinfo or hwmonitor to check those temperatures, RAM sensor might be available depending on motherboard, but in case it's not just get into your case and feel it with your hand, carefully)
also, check that your windows power settings is set to maximum/full power profile. and that you don't have any other power saving programs that might affect performance.