Hi and thanks for the heads up! What version of Mac OS are you using? Try hovering your mouse on the upper right corner of the screen, it should reveal a button to play the game in windowed mode. I'm on a early 2014 MacBook and it works.
OS X 10.11.3. Completely forgot about the fullscreen maximize button! Performance goes back to normal in a small enough window, but FPS issues persist even when the window is sized at about 75% of the screen. May be a Retina issue or a problem with Intel Iris graphics, perhaps.