It kinda depends on where in Plum, where in Pittsburgh, and what time of day. Most likely one would come into Pittsburgh from Plum via I376 and have to pass through the Squirrel Hill Tunnels which are pretty much right at the city boundary. During rush hour, for what ever reason, cars just can't seem to maintain their speed through the tunnel ...... So there is always a huge jam up at that point.
Getting into the city during rush hour is a challenge from most any direction because we have tunnels and bridges to navigate from all compass points. So the lucky commuter is the one who can be flexible with their travel time. If you have to be Downtown between 8:30 and 9 (like thousands of others), then your commute time is easily doubled from what it would be if you arrived between 7 and 7:30, for instance.
So shifting your commute times will do much more for you than living a little closer in.