You’re in a Ferrari, on the Autobahn. Happy at 40mph? 60? 100? Your car is capable of 200mph and there are no speed limits. If you’re not going to floor it, take the next exit.
If your art director gives you more work than two designers can realistically handle, get it done anyway. It’s a state of mind. You can do two people’s work. Of course you can. Plan properly and you won’t even have to work very hard. Find a better way. Invent a shortcut. Design a system. Revolutionize your industry. Why not?
No, that is not part of your job, but doing only what you are getting paid to do is a sorry excuse for a life.