Performance and skill are two complementary terms. How can you perform well without any skills? Similarly you may have skills but can you perform well?
In the infantry shooting a rifle well is one of the chief tasks, but anyone can tell you that you must master the skill of shooting before you can perform well. If you have mastered the skill of shooting, you may not perform well under stress such as when there is an enemy who is shooting back at you.
As an air force mechanic, you have the skills but not necessarily the performance to match. For example you may be overly meticulous and cautious in doing all your tasks but not perform as well as other because you're too slow. Many repairs tasks are based on the average time to complete the repair task. If you take more time and use more materials in comparison to the other technicians who successfully completing the same task but who are on time, then you are not performing as well.