An update addressing a performance issue in Captain Scarlett and her Pirate's Booty content with PhysX set to Medium or High was released on October 25th: http://support.gearboxsoftware.com/entries/22126761-pc-update-information
You can download the latest Borderlands 2 update by restarting Steam. All updates are cumulative; you only need to install the most recent version to receive all updates.