Are you sure? I'd think it isn't passing function calls, but rather direct hardware access. I might be wrong.
Either way, there's clearly a way to make the game run significantly faster on OSX, but Valve isn't doing it for some reason.
They use the platform graphics APIs like any other app.
Parallels probably work by decompiling DirectX shader bytecode back into GLSL, which is already optimized for OS X shader compiler, therefore there is less burden on the optimizer.
Another example of poor drivers are various GLES drivers on mobile, mostly trivial optimizations can drastically increase the performance: http://aras-p.info/blog/2010/09/29/glsl-optimizer/ and https://github.com/aras-p/glsl-optimizer
i can't really give you an answer on this, but if i had to guess i'd say it works better on anything but radeon cards(but then again are radeon cards supported on macs nowadays?)
also, not exactly the same, but still interesting measurements.
http://www.phoronix.com/scan.php?page=article&item=ubuntu130...
valve worked closely with nvidia and intel to improve the performance and compliance of the drivers. check this out for example http://blogs.valvesoftware.com/linux/faster-zombies/
I'm using the NVidia latest stable drivers (from the NVidia website), which were a pain to install under *buntu.
It actually seems a lot smoother running natively compared to running under Wine. I can't put my finger on it. The frame rate under Wine seems OK. The image looked fine. But it didn't "feel" right.