Poor nutrition . The food one eats is the fuel your body needs to perform. As an athlete who wants to perform at a high level, it behooves you to put the best fuel possible in your body. Fast food is one of the worst fuels possible. Consider less processed foods and more natural whole foods.
No one. No one, athlete or otherwise, can eat fast food without consequences. We all are " a consequence" of what we eat and what we do with the energy we derive from eating. It isn't how fast the food is made but rather what is in the "fast food" that you eat.