This MacNN thread notes and interesting inactive feature of OS X 10.1, posted by moki (Andrew Welch): The window server has a cool feature in OS X 10.1 that isn't enabled by default (though it will be in an upcoming update, as I understand it): window buffer compression. later.... So what Apple did was they implemented a compression mechanism into the window server. When a window's contents haven't changed for a given period of time, the window server compresses them, so they take up less memory. Since it uses a compression method that doesn't require the buffer to be fully decompressed to do compositing (dragging a window around, updating the screen, etc.), you won't notice a slowdown with this compression turned on. In fact, because less memory is being used up by the window buffers, more RAM will be available for your applications, with will mean less virtual memory paging, and may in fact result in speeding up your machine. Additionally, since less data needs to be read (it is compressed, after all!), things like updating windows may be faster as well.