The site used a late-2009 MacBook Pro and a custom-built hackintosh for testing.
We can see immediately that Portal for Mac is nowhere near as capable as its Windows counterpart. However, there are several interesting points to take away from this. Portal for Mac, in its current form, is at least able to deliver playable frame rates, even on a lower-end Mac like last year's MacBook Pro. Admittedly, the biggest bottleneck here is the GPU.
On the hackintosh side, it looks like the Mac drivers play a key role. "Until Apple comes out with higher-performance drivers, we'll continue to see a significant delta between the two platforms." TomsHardware also notes that OpenGL is better suited to productivity than gaming.