I've written a patch for Mac OS X/Carbon version of emacs 21-3. It's
a rewrite of the event handling code to have the UI placed in a
separate thread from the lisp eval loop; the main benefit is that on
Jaguar, C-g will break synchronous processes. It is preliminary, so
although emacs will rarely hang, it will often crash. It also
increases UI responsiveness of emacs and reduces CPU usage. The main
problems have to do with scrolling and menu handling. I would
appreciate it if Mac Emacs developers with both 10.1 and 10.2 tried it
out and let me know any other problems, and hopefully fix the bugs.
The web page below has the link to the patch and a fair amount of
explanation about how the patch works.