Ubisoft had a pretty simple idea for this game: sell the ported version on release and patch later. This lead to various issues, including a buggy release on the PC.
That said, did you look at the system requirements (better yet, the recommended requirements)? They are very high...which again has to do with the fact that they simply ported it to the PC (as oppose to optimise the game for the PC). If you do meet these requirements, I would highly recommend re/installing the latest GPU drivers. More importantly, make sure to install the game patches which should by now improve your performance a lot.
If I may take this opportunity to give my personal opinion here. This game was very disappointing. Even if you did fix your performance issues, the gameplay mechanics and the poor choice of design really spoils the entire game. Fix and Play at your own risk.