Ok let's see, I might be lacking info here, but do you have a 2 GB video card? If thats the case it could be one of the reasons. I have a 1GB :/
Second, Mafia III is horrible optmized, just check the reviews, I had a I5 4690k and GTX 970 in the past and still couldn't achieve 60 fps stable most of the time on maxed out settings. Hmm, Weird
Third, GTA V and Overwatch are very well optmized games, even low spec computers can run at decent settings at high framerates.
Fourth - Try disabling Anti Aliasing setting in Mafia III, I don't know in which settings are you playing the game at but this seems to be the most fps killer. I am running it on lowest settings possible