I do not guarantee that this will work for you, but it worked for me and improved my performance to where it was playable at 60 fps. All you have to do it go to MKX in your Steam library and right click on it. Click properties and click set launch options. In the box provided, type this exactly (minus the quotes):

"-dxlevel 90 -high"

What this does is forces the game to run in dx9 which improved my performance, and also allows your system to allocate more power to the game.

Hope this helps, and have fun playing some Mortal Kombat :D

EDIT: I've tested and I've seen no difference in performance from forcing the game to run in DX 9, if it even runs it. Just put "-high" and the game will still have more memory to run off of.