Hi. I have the same problem.
I had been playing CS:S on my Macbook (aluminium unibody, late 2008 version, nvidia 9400 graphics board) on OS X Leopard 10.5.8, CX Games 9.0. CS:S had approximately 49-50 FPS with the default settings the game came up with on a 1680 x 1050 external display.
Today I installed Mac OS X 10.6.3 from scratch (**backed up everything and formatted the disk), and have a clean install. After restoring my bottles from the archives and started playing I noticed a 50% slowdown. On the same resolution, same video settings for the source engine, I get only between 23 and 24 FPS.
Do we have to modify anything or is this a known problem for 10.6.3? Any ideas to troubleshoot? Most likely the new drivers are to be blamed, but wanted to be sure.
Thanks.
//
Carlos.