It appears that Dell's most recent BIOS releases for the XPS 15 9570 have caused a GPU bug in some devices. Originally reported on Reddit, some XPS 15 9570 laptops power limit throttle their NVIDIA GPUs until they hit a temperature threshold, causing noticeably worse performance in games.
https://www.notebookcheck.net/Some-Dell-XPS-15-9570-laptops-may-have-a-BIOS-related-GPU-bug.355026.0.html