I realized something unpleasant but important: The speed of my game was not limited by my timer but by my CPU. That means that other Macs can work very, very differently. In order to fix that, I fixed the worst bottleneck (Immediate Mode code). The whole game went VA-VA-VOOM and got totally unplayable - but really smooth. So now I have to re-tune the whole mess.

I knew all along that Immediate Mode wasn't an good idea, but the difference was bigger than expected.