I've been noticing this problem for a bit and figured it'd get fixed but 
it hasn't so here is a bug report.

With the Emacs 25 pretests, it takes waaaaayyy too long to edit some C 
code. To reproduce the problem use the attached file (taken from the 
Emacs source code) and run:

emacs -Q config.h
M-x goto-line RET 1661 RET / /

On my six-year-old desktop the second '/' takes about 10 seconds to 
echo.  This sort of thing makes Emacs effectively unusable for editing 
config.h.


In GNU Emacs 25.0.92.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.7)
  of 2016-03-02 built on penguin.cs.ucla.edu
Repository revision: 100346aa226e4eacc56f390c099bb9aab585b5f4
Windowing system distributor 'Fedora Project', version 11.0.11800000
Configured using:
  'configure --enable-gcc-warnings'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
   value of $LANG: en_US.UTF-8
   value of $XMODIFIERS: @im=ibus
   locale-coding-system: utf-8-unix

Major mode: C/l

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
   abbrev-mode: t

Recent messages:
config.h has auto save data; consider M-x recover-this-file
Mark set
You can run the command ‘goto-line’ with M-g g
Mark set
Undo!
Mark set
Undo!
Mark set
Mark saved where search started
Auto-saving...done

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec epg epg-config gnus-util mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util help-fns mail-prsvr mail-utils character-fold
misearch multi-isearch cl-extra help-mode cc-mode cc-fonts easymenu
cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
cl-loaddefs pcase cl-lib time-date mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win
term/common-win x-dnd 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
dbusbind inotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 123955 4665)
  (symbols 48 21742 0)
  (miscs 40 57 290)
  (strings 32 28264 16097)
  (string-bytes 1 748499)
  (vectors 16 14562)
  (vector-slots 8 502379 4556)
  (floats 8 166 20)
  (intervals 56 4059 0)
  (buffers 976 12)
  (heap 1024 29227 2318))