But if using these downgraded settings is frustrating, or you're still getting abominable frame rates, here's a few workarounds which I've been using. I'm running a MacPro 1,1 with the Nvidia 8800 GT and OSX 10.6.4, and have had the same massive drop in frame rates many users have reported.
[note-- one user reported no difference after resetting their resolution, so your milage on this trick may vary]
1) When loading into a new level, hit F10, change your resolution to something lower and hit accept. When it asks for confirmation, press No and go back to your original resolution. Performance should be restored to what it should be, and generally this should last you through the entire level.
2) After finishing a level, when you're on the Hyperion load screen, hit Command-M to toggle to windowed mode. The cut scene should be watchable, if smaller.
Just a note on the side, here's a tip clipped from a blizzard thread that can reduce some of the strain on your video card, particularly if it's a bit on the old or dusty side.
Certain screens make your hardware work pretty hard
Screens that are light on detail may make your system overheat if cooling is overall insufficient. This is because the game has nothing to do so it is primarily just working on drawing the screen very quickly. A temporary workaround is to go to your Documents\StarCraft II\variables.txt file and add these lines:
Hope this is helpful to those who haven't been able to revert to 10.6.3 and are struggling with performance issues. Here's hoping for update drivers soon!
Update: Crappykiller reports that when running at a resolution other than the native desktop resolution, the same reset effect can be achieved by switching to windowed mode and back. For those playing at the native resolution, switching to windowed mode mid-level can cause a reduction in frame rate, however.