There is apparently some bad interaction between Emacs 23 and the Mac
OSX desktop with respect to output from background processes.

When emacs is processing such output, there is a kind of lock up where
it literally can take many seconds to switch desktops or to switch
between applications (Cmd-TAB).

I have attached a small example file that exhibits the behaviour. It
creates a number of frames and starts a compilation process (simply a
"ls -lR /System" command) that generates quite a lot of output. Once
that process is running, attempts to switch desktops or to switch
between applications takes very long time. Apparently the severity of
the problem is related to how many frames are created. With the test
file as attached it is really bad, if no extra frames are created, it is
barely noticeable.

The tests I have done just now is on Mac OSX 10.6.2 on an Intel based
Mac Book Air but I have seen the same behaviour on my powermac G5. If I
compile emacs to use X11, there is no such problem.

The emacs identifies itself as "23.1.94.1" and was taken from the
repositorys emacs23 branch a few days ago. The configure options used
when building was just --prefix, --with-ns and --without-dbus.

Let me know if there is any more information I can provide.