Qt runs cycle-count identically on GNOME vs KDE, although some distros do a better job than others cross-theming everything (Fedora does well, Ubuntu does not). The concept of people actually using OS X is new enough that it's probably not as well optimized, although it's apparently a priority for 4.6.
As for games not working, I seem to not see anything in your posting history where you ask for help getting them to work. Ditto on help configuring SDLMAME for best speed. I can't help if you don't say anything.