bug#40007: 28.0.50; Emacs gets very slow when displaying a long line

[Jan Synacek <jsynacek@redhat.com>]

1) emacs -Q
2) M-x shell
3) execute a program that displays a single line, about ~193000
characters long

I accidentaly wrote a scheme program that dumped an entire file as a
single line, about 193k characters long, into the shell buffer. Emacs'
reponse latency becomes *very* slow, especially when navigating the
point over the long line.


In GNU Emacs 28.0.50 (build 7, x86_64-pc-linux-gnu, GTK+ Version 3.24.11, cairo version 1.16.0)
 of 2020-03-10 built on jsynacek-ntb.brq.redhat.com
Repository revision: a98c8f5a098cf646c282be67cce9fb7999d353d7
Repository branch: master
Windowing system distributor 'Fedora Project', version 11.0.12006000
System Description: Fedora 30 (Workstation Edition)

Recent messages:
Loading /usr/share/emacs/site-lisp/gtags.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...
No match [2 times]
Buffer "*shell*" has a running process; kill it? (y or n) y
next-line: End of buffer
Revert buffer from file /home/jsynacek/todo.org? (y or n) y
Making completion list...

Configured using:
 'configure --prefix=/home/jsynacek/emacs'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD
PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Org

Minor modes in effect:
  shell-dirtrack-mode: t
  delete-selection-mode: t
  show-paren-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug sendmail shell org-element avl-tree
generator ol-eww ol-rmail ol-mhe ol-irc ol-info ol-gnus nnir gnus-sum
url url-proxy url-privacy url-expand url-methods url-history mailcap shr
url-cookie url-domsuf url-util svg xml dom gnus-group gnus-undo
gnus-start gnus-cloud nnimap nnmail mail-source utf7 netrc nnoo
parse-time iso8601 gnus-spec gnus-int gnus-range message rmc puny rfc822
mml mml-sec epa derived epg epg-config mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader gnus-win gnus
nnheader gnus-util rmail rmail-loaddefs rfc2047 rfc2045 ietf-drums
text-property-search mail-utils mm-util mail-prsvr wid-edit ol-docview
doc-view jka-compr image-mode exif dired dired-loaddefs ol-bibtex bibtex
ol-bbdb ol-w3m org ob ob-tangle ob-ref ob-lob ob-table ob-exp org-macro
org-footnote org-src ob-comint org-pcomplete pcomplete comint ansi-color
ring org-list org-faces org-entities time-date noutline outline
easy-mmode org-version ob-emacs-lisp ob-core ob-eval org-table ol
org-keys org-compat advice org-macs org-loaddefs format-spec find-func
cal-menu calendar cal-loaddefs gtags server sh-script smie executable
edmacro kmacro delsel paren info package easymenu browse-url
url-handlers url-parse auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json subr-x map url-vars seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer 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 composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray 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 threads dbusbind inotify lcms2 dynamic-setting
system-font-setting font-render-setting cairo move-toolbar gtk x-toolkit
x multi-tty make-network-process emacs)

Memory information:
((conses 16 170604 23396)
 (symbols 48 19519 1)
 (strings 32 64980 1639)
 (string-bytes 1 2222006)
 (vectors 16 30280)
 (vector-slots 8 349836 16932)
 (floats 8 197 86)
 (intervals 56 414 0)
 (buffers 1000 14))