Aside from that, I've found that generally in my life, I can only do three things well at any given time. Health, work, relationships, other goals... It takes mental energy to make decisions and process things, and even you are still spending the same amount of time on something. So yes, you can spend your time elsewhere, but the fact that you have a second full time job is highly correlated with you doing your first one worse. Are you claiming that your productivity at work is going to be the same if you have a side project or not? I definitely think there's a clear difference, having done it at one time.