MacOS X blit really slow

I’m seeing terrible frame rates for SDLFlip() and SDLUpdateRect* under Mac
OS X – about 9-12 FPS. I’ve tried various combinations of SW/HW,
fullscreen/windowed, 16/32 bit, UpdateRect/SDLFlip/UpdateRects. All of my
surfaces are created with DisplayFormat(). UpdateRects() only gained about
+5 FPS. The same code runs at 70+ FPS on a relatively equivalent PC. Is
this the expected behavior under Mac OSX? Are there any Mac OSX-specific
modifications I should make? Thanks. -John