Emacs in particular, and a lot of Unix software in general, run very poorly in Mac OS X compared to Linux or OpenBSD. I had to look up what vfork was, and learned that it was deprecated in POSIX 2008. Not surprising. Mac OS X seems to be in perpetual beta mode, where it is always half-way behind on basic Unix features, and the other half are proprietary interfaces that get deprecated/broken/incompatible changes every major release. Switch to a better operating system.