I am experiencing the same issues with Sculptris on a Mac. Paint Mode is basically inoperable. It doesn't matter how low my mesh count is or how low I set my resolution before going over to paint mode. It simply takes minutes to rotate or zoom, and the actual paint functions don't work.
I would love an update or a patch for Macs. I have been watching videos from digital-tutors and they are painting on high res meshes over 2 million tris and it is running smooth and quick on a PC.
I have a dual boot Laptop with Windows, I will install Sculptris on the PC partition just to rule out hardware and see if it is a problem limited to Operating System.