Too much anxiety can negatively impact athletic performance. For instance, if you have a panic attack right before your race, then this will severely impact your performance, and you may even become paralyzed to act.
On the other hand, if you have no anxiety, then this will also negatively impact your athletic performance. For instance, if you are so relaxed that you are almost comatose, then this will severely impact your performance, and you become paralyzed to act.
So if you want optimal athletic performance, then you need a moderate level of anxiety...not too much, and not too little. The following graph illustrates how anxiety and stress affects athletic performance: