Here is an engineer outputting X amount of work for a given work day which is equivalent to his co-worker only that he does it in less time. If you then require

Here is an engineer outputting X amount of work for a given work day which is equivalent to his co-worker only that he does it in less time. If you then require that person to work more than X in order to fill his hours, he will be incentivised to work less as he will find it to be unfair that he has to do more than his colleges. Thus, putting down time on his own projects should not be seen as a negative thing. Mainly because he is actually improving his skillset which in return benefits the company he is working for. This however presume that he is in fact producing as much as his colleges in less time. The only two options I see is to let it continue as it is nothing wrong or cut him some deal where he can work less hours OR grant him a raise and ask him to do more for more pay. It all comes down to expectations. If that employee meets your expectations then there is no reason to fire someone who is doing well just because he is better and more efficient than everyone else!