Re: M-x grep is too slow

[Richard Stallman]

    Something has made `M-x grep' unbearably slow.  With 'emacs -Q' this
    command

       M-x grep -nH -e while emacs/lisp/gnus/*.el

    took over 20 minutes to complete -- I lost patience after that and
    killed Emacs, so it might actually take hours.  For comparison the
    same command takes 2-3 s in Emacs 21.3 and 1 s on the command line.

In what function is it spending most of its time?

You can set debug-on-quit to t and try typing C-g a few times,
and save the backtraces, then continue.  I doubt it will take
more than a handful of times before you see the pattern.