bug#895: slow processing of process output

[Markus Triska <markus.triska@gmx.at>]

I can reproduce the problem on OSX, and could significantly improve the
running time by setting process-connection-type to nil before doing the
rgrep. Let rgrep.el consist of:

   (grep-compute-defaults)
   (setq process-connection-type nil)
   (rgrep "emacs" "*.el" "~/emacs/lisp/")

   (while (process-status "grep")
     (sit-for 0.1))

and do:

   $ time emacs -Q --script rgrep.el

Then contrast this with retaining t for process-connection-type.

I have seen that using a pty can be a major performance problem also
when communicating with other external programs, and I recommend to
let-bind it to nil before starting the process when a pipe suffices.