MLAA is being produced in addition to whatever other AA mode you have selected. The setting doesn't override the othe MSAA settings in the control panel or in game. The game will render with whatever AA mode you've told it to and then the MLAA will go to work on that output. If you want to see the difference in performance between MLAA and other modes you have to set the regular MSAA to Application select and turn AA off in game. That will give you just the MLAA performance and image quality. From there you can decide what gives you the best image quality at an acceptable framerate. But if you have MSAA on and then you enable MLAA it will always make performance go down because you are making your vidcard do additional processing on each frame.