To me it sounds like the fact that you're "slacking" for five hours a day is what enables you to complete your work effectively during the other three hours. I

To me it sounds like the fact that you're "slacking" for five hours a day is what enables you to complete your work effectively during the other three hours. I expect that if you forced yourself to slack less and do more hours of "real work", you would get less done in more time, and with degraded quality. If you can change your perspective and feel less guilty, I recommend you keep doing what you do for both your sake and your employer's. It's a long term win-win.