with same graphics same resolution ???
Assuming you have installed it correctly plugged pci-e cables and installed drivers probably your game is now running at another graphics, maybe if you lower at the same point you were using your 5450 you will get same graphics.
Maybe you aren't using the Pci Video card, make sure your monitor is connected to the video card slot, and that your game is set to run on it, maybe you are running with onboard video card.
i would say, check graphics probably you were running at much lower graphics before, and now the game is lagged because you are running at higher settings.