Explain to me how is this even possible or how does it work?

i got 3070 and 7 5800X3D for some reason if i play this game for literally an hour or 2 my fps gets lower permanently and not just in this game but across all games my fps is lower until i restart PC i was playing other games for past months i never had anything like this but the moment i play last epoch for few hours i get this issue it doesnt make sense how is this even possible?