600 fps is not so much, and every computer should be able to do this, specially if you render a black screen.
except of course if you sync the rendering on the screen, so you can't do more than 60fps.
this is default on osX.
so, when asking 600fps, the GPU spend most of it's time waiting for a syncro.

And the entire GUI on a Mac is OpenGL so that is why even the mouse pointer doesn't draw well. I guess DX on Windows would do the same thing these days.

Maybe setting the fps to higher than 60Hz should turn off vsync...