I was playing for a day or two and all of a sudden my game slowed down. Like lag. When i went to france the game ran smooth again. Coming back to Bridgeport, again with the lag. It looks like i don't have enough ram but that shouldn't be the case.
A lot of people on the official boards are saying it's because of junk left behind by NPCs (such as abandoned cars). There's a mod people are recommending that cleans up that stuff, though I haven't tried it yet myself.