If you're only getting 70-90 FPS there must be something badly broken with your iMac. The thing should be able to run at 125 FPS (capped) on high res without breaking a sweat.
Yeah I agree, I did quite a bit of looking into it as well. In bootcamp I was able to manage a solid 125fps, yet when in OSX it varied between 70-90.
Although I performed many tests I was never able to get the bottom of it.
Apples OpenGL drivers are known to have sub par performance compared to those for Linux and Windows.
That image is not from a Pi
Fair enough.
Check if your iMac defaults to software rendering for some reason.