Having it turned on, will lock you to 30FPS. Having the app continuously running at 60FPS can take quite the toll on the device in terms of performance, heat and battery consumption.
Bc with the option always on the game always perform at the same fps (30fps) but with the option disabled the game run well (60fps) but if there is a B772 or a A350 displaying on the screen the game lags a bit (45fps)