bug#895: slow processing of process output

[Richard Copley <rcopley@gmail.com>]

I tested in current master with mingw-w64 on Windows. The rgrep
command given in the OP takes 29 s in Emacs, but the corresponding
shell command[1] takes over a minute in Windows cmd.exe (at a guess,
that's because cmd.exe scrolls line-by-line so has to push a lot of
pixels).

Likewise, building Emacs is faster with "M-x compile" than it is on
the shell (from a clean checkout on my configuration it takes 7 min 43
seconds in Emacs and 9 minutes 15 seconds in cmd.exe).

I'm tempted to conclude that this bug has been fixed.

[1] G:\usr\bin\find.exe . -type d "(" -path "*/SCCS" -o -path "*/RCS"
-o -path "*/CVS" -o -path "*/MCVS" -o -path "*/.src" -o -path "*/.svn"
-o -path "*/.git" -o -path "*/.hg" -o -path "*/.bzr" -o -path "*/_MTN"
-o -path "*/_darcs" -o -path "*/{arch}" ")" -prune -o ^"^!^" -type d
"(" -name ".#*" -o -name "*.o" -o -name "*~" -o -name "*.bin" -o -name
"*.bak" -o -name "*.obj" -o -name "*.map" -o -name "*.ico" -o -name
"*.pif" -o -name "*.lnk" -o -name "*.a" -o -name "*.ln" -o -name
"*.blg" -o -name "*.bbl" -o -name "*.dll" -o -name "*.drv" -o -name
"*.vxd" -o -name "*.386" -o -name "*.elc" -o -name "*.lof" -o -name
"*.glo" -o -name "*.idx" -o -name "*.lot" -o -name "*.fmt" -o -name
"*.tfm" -o -name "*.class" -o -name "*.fas" -o -name "*.lib" -o -name
"*.mem" -o -name "*.x86f" -o -name "*.sparcf" -o -name "*.dfsl" -o
-name "*.pfsl" -o -name "*.d64fsl" -o -name "*.p64fsl" -o -name
"*.lx64fsl" -o -name "*.lx32fsl" -o -name "*.dx64fsl" -o -name
"*.dx32fsl" -o -name "*.fx64fsl" -o -name "*.fx32fsl" -o -name
"*.sx64fsl" -o -name "*.sx32fsl" -o -name "*.wx64fsl" -o -name
"*.wx32fsl" -o -name "*.fasl" -o -name "*.ufsl" -o -name "*.fsl" -o
-name "*.dxl" -o -name "*.lo" -o -name "*.la" -o -name "*.gmo" -o
-name "*.mo" -o -name "*.toc" -o -name "*.aux" -o -name "*.cp" -o
-name "*.fn" -o -name "*.ky" -o -name "*.pg" -o -name "*.tp" -o -name
"*.vr" -o -name "*.cps" -o -name "*.fns" -o -name "*.kys" -o -name
"*.pgs" -o -name "*.tps" -o -name "*.vrs" -o -name "*.pyc" -o -name
"*.pyo" ")" -prune -o  -type f "(" -iname "*.el" ")" -exec
G:\usr\bin\grep.exe --color=always -i -nH -e "emacs" {} +


In GNU Emacs 25.0.50.1 (x86_64-w64-mingw32)
 of 2016-01-02 built on MACHINE
Repository revision: 372d00a981942a5da868a33372eb29d5806f2ab3
Windowing system distributor 'Microsoft Corp.', version 10.0.10586
Configured using:
 'configure --prefix /c/emacs/emacs-20160101-122333
 --without-imagemagick --disable-dependency-tracking
 --enable-locallisppath=%emacs_dir%/../site-lisp 'CFLAGS=-Og -g -ggdb''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: ENG
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message dired format-spec
rfc822 mml mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util help-fns help-mode easymenu cl-loaddefs pcase cl-lib mail-prsvr
mail-utils time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel dos-w32 ls-lisp disp-table
w32-win w32-vars term/common-win tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote w32notify dbusbind w32 multi-tty
make-network-process emacs)

Memory information:
((conses 16 83465 5112)
 (symbols 56 19008 0)
 (miscs 48 36 86)
 (strings 32 14081 5015)
 (string-bytes 1 386417)
 (vectors 16 11288)
 (vector-slots 8 408630 5027)
 (floats 8 139 4)
 (intervals 56 239 22)
 (buffers 976 11))