I have an XPS 15 that originally would run games perfectly but something recently happened where now the performance that happens in the game is a lot worse. I can't run CSGO above about 24 frames and if I go to full screen on a game like terraria I even start to lose major frames. When I check the task manager there seems to be nothing that is capping out and GPU usage is about 20% while CPU is around 50%. Memory is fine as well. What could be the issue because it use to run fine?
Check your temps while gaming. Check all ventilation slots and make sure they are free of dust/debris. Run a synthetic benchmark like https/benchmark.unigine.com/heaven and see how it performs (basically to see if it's a game or hardware issue)