You only need to improve when you don't have anything to loose. For example if you are not the employee of the year, then you have a lot to work on. Improving your performance is not always the best thing to do. You might feel the urge to want to improve but sometimes what you give is enough to proof that you are the best at what you do. You just have to be efficient at you do.
Improving your performance is a risky task. Improving means you will have to do things different and sometimes the environment you work for does not need that change right now. For that you must know how to maintain performance. This is even more challenging. A good way to maintain performance is to keep doing what you are doing right now to satisfy the objectives of your group or organization.
So in the war of improving and maintaining performance, most of the times it is better to only maintain performance rather than improving it. Unless you work for a very competitive organization then things might be different.