-It's designed specifically to improve performance
-It can be repeated a lot
-Feedback on results is continuously available.
-It's highly demanding mentally.
-It isn't much fun.
If it seems a bit depressing that the most important thing you can do to improve performance is no fun, take consolation in this fat: It must be so. If the activities that lead to greatness were easy and fun, then everyone would do them and they would not distinguish the best from the rest. The reality that deliberate practice is hard can even be seen as good new. It means that most people won't do it. So your willingness to do it will distinguish you all the more.