Subject: bug#28795: 25.2; UTF-8 language environment causes extreme sluggishness

From: Eli Zaretskii <[email-masked]>
Date: Thu, 12 Oct 2017 12:31:22 +0300

> From: Mark Laws <[email-masked]>
> Date: Thu, 12 Oct 2017 18:15:53 +0900
> 
> M-x set-language-environment UTF-8

On Windows, doing this is a mistake (but not because it causes
sluggish redisplay).

> M-: (insert #x2207)
> 
> Then hold down C-o until you have a bunch of empty lines, and start moving
> around the buffer with the usual movement keys.  You will notice it is very,
> very slow.  Then delete the character from the buffer, and notice how things are
> fast again.

I cannot reproduce this here.  What font is used to display that
character?  You can find this out with "C-u C-x =" with the cursor on
the character.

On my system, I have the Symbola font installed, which is used for
displaying this character, and the problem doesn't happen.

As another thing to try, set inhibit-compacting-font-caches to a
non-nil value, and see if that helps.