Cant play the King Raven mission because of lag

Soon as it starts up its all choppy and the sound is stuttering,so how am i supposed to cut the legs off of something I cant even see because the game isnt running smoothly? Seriously…Played the whole campaign and now all of a sudden the game wont run. I am not even trying to play online,just the single player campaign and this pile wont run.

Xbox or PC?