bug#28535: 25.2; Composed charater colon slows down Emacs performance while you in buffer with this character

[Eli Zaretskii]

> From: Oleksandr Gavenko <gavenkoa@gmail.com>
> Date: Thu, 21 Sep 2017 13:35:28 +0300
> 
> With composed two colon characters Emacs become extremely slow.
> 
> I figured that out when enabled cfengin3-mode and two colons :: was shown as
> single composed character.
> 
> With runemacs -Q it slows down when you cross that character by moving
> pointer.

Please provide a simple reproduction recipe, for reference, if you
can.

>                position: 110 of 136 (80%), column: 7
>               character: : (displayed as :) (codepoint 58, #o72, #x3a)
>       preferred charset: ascii (ASCII (ISO646 IRV))
>   code point in charset: 0x3A
>                  script: latin
>                  syntax: .    which means: punctuation
>                category: .:Base, a:ASCII, l:Latin, r:Roman
>                to input: type "C-x 8 RET 3a" or "C-x 8 RET COLON"
>             buffer code: #x3A
>               file code: #x3A (encoded by coding system undecided-dos)
>                 display: composed to form "::" (see below)
> 
>   Composed with the following character(s) ":" by the rule:
>       (?∷)
>   The component character(s) are displayed by these fonts (glyph codes):
>    ∷: uniscribe:-outline-MS 
> Gothic-normal-normal-normal-mono-13-*-*-*-c-*-gb2312.1980-0 (#x49D)
>   See the variable ‘reference-point-alist’ for the meaning of the rule.

MS Gothic is known as a trouble-maker in this context.  Can you try
installing Symbola?  Doing that might eliminate the problem.  Emacs
will automatically use Symbola, if installed, for such characters.

Another possible remedy is to customize inhibit-compacting-font-caches
to a non-nil value.