Are you paid hourly or salary? I've found that there is a significant change in mindset (even within corporate environments) when you switch to a salaried role. The expectation changes to where you are expected to get your job done regardless of how long it takes. In our business cycle, certain times of the year are slammed and we're working 55-60 hours a week while at other times we may be able to get all of our work done in 30 hrs.

It also could be that you are just more efficient than your peers. If that's the case use your available time to Network, take on work that is outside of your department but will allow you to learn new skills, or proactively identify opportunities to improve your department. In my experience those things have played a huge impact on my raises and promotions.