As promised, I re-downloaded the Windows versions of Team Fortress 2 and Portal and ran some framerate comparisons.
In Portal running on Vista under Boot Camp, I get a fairly steady mid 20s to low 30s FPS at 1680×1050, using the recommended settings: model detail high, texture detail medium, shader detail high, water detail set to reflect world, shadow detail high, colour correction enabled, no antialiasing, 8X anisotropic filtering, vertical sync disabled, motion blur disabled, HDR on full and bloom off.
TF2, presumably running on the same Orange Box engine, unsurprisingly gets a very similar frame rate with the same settings. I have field of view set to 75. Obviously it can drop a little in particularly busy fights, but with a decent ping it never becomes unplayable. With the same settings on OS X, I’m getting about 10-12 FPS, occasionally peaking at about 15, and regularly dropping to low single digits when the fight gets crowded. To my mind that is unplayable. The recommended settings on the Mac version are lower; with the game suggesting low shader detail, medium shadow detail and trilinear filtering. Using those setting gained me, perhaps, a single frame every second.
With the same settings as under Boot Camp, the Portal Mac beta frame rate drops to a similar level. In narrow corridors it can reach the low twenties, but in large rooms, or as soon as I start placing portals, it falls shockingly low. If you’ve ever experimented with the Source engine’s frame counter you’ll know that, as well as displaying actual numbers, the text is displayed in traffic light colours. In the Windows games it appears to stay red up till around 35, and doesn’t go green till nearly 60. Under OS X it turns green at about 28-30. It seems Valve have decided that a lower frame rate is acceptable on the Mac, which is disappointing.