Laggy on high pc?

My pc is pretty good its my second day first day i had the same problem but it was gone a bit later now i am having the same problem lag its normal when i reduce the resolution rn i am on fullscreen(real) and resulation 2560x1440 and its 30 fps why is this too laggy and have fps problems rn i know the game is stuck on 60 fps but 30?