Thanks for mentioning this, I'm seeing it on my windows machine as well.
I think I see the problem. To maintain 60FPS, the game sleeps for the remainder. At least on my windows machine, when it does this, it sometimes sleeps for way too long (even though the requested sleep time is correct). This is what's behind it being at 30FPS on your machine. Very strange, I'll have to put in a better system.