If you cook a brisket too slowly, it will become overcooked and dry. The best way to avoid this is to cook the brisket at a moderate temperature, between 325-350 degrees fahrenheit. This will ensure that the brisket is cooked evenly and remains juicy.if you cook a brisket too slow, the meat will become tough and dry. The connective tissue will not have enough time to break down, leaving the meat chewy. The fat will not render properly, leaving the brisket greasy.