I don't have any particular advice about getting it to run faster, but you can try this to reset it back to working:
In the Steam GUI, right-click on CS:S and select Properties from the context menu. In the properties dialog, there's a button to set the game launch options. You need to have "-novid" in there to work around a bug in CrossOver. To reset the graphics settings of the game, you should also add "-dxlevel 81".
Run the game once with that setting. It should come up fine (no "mode not supported" window). If it still fails, then I'm on the wrong track.
After you've run it once like that, you should remove "-dxlevel 81" from the launch options because that will interfere with any video settings you try to save from within the game.
I hope that helps. Good luck.