Not long after the software update from Apple, performance issues concerning the Mac OS X Snow Leopard 10.6.4 Update and certain Steam titles cropped up for some users.
10.6.4 performance issues affect Macs with Nvidia GPUs (includes Macbooks, Macbook Pros and several iMac models).
Steam is actually recommending holding off updating to 10.6.4 while it communicates with Apple for a solution. Here’s Steam’s official notice for mac users.
The recent 10.6.4 update from Apple has noticeable performance issues for NVidia graphic chip owners running high performance games. If you wish to avoid this, you should consider waiting to install the 10.6.4 update until Apple has had the opportunity to address this issue.
If you have already installed this update and believe your graphic performance is affected, please contact Apple support (http://www.apple.com/support/) for details on what to do.