It doesn't really matter how long the game is or if the game is "competitive" or not, sorting by IGT allows for anyone running to be on the same playing field as everyone else regardless of PC specs (or internet connection in the case of Co-op runs). Sorting by IGT is the standard for most PC games, and is the standard for the entire series (including BL1 Enhanced, which is even less "competitive" than BL3). IGT will always be the primary timing method.