bug#21028: Slow font rendering in emacs

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 21028@debbugs.gnu.org
> From: Ralf Jung <post@ralfj.de>
> Date: Mon, 13 Mar 2017 19:12:10 +0100
> 
> > Do you have any measurements that would show the slowdown in numbers?
> 
> I know very little about emacs, so I wouldn't know how to make any
> measurements.

If some reaction time is slow, just estimating how many seconds it
takes might be good enough, if you describe the slow command and what
is on the screen when it is slow.

> > Also, what is your value of system-configuration-options and
> > system-configuration-features?  And which version of Emacs is that?
> 
> Sorry, I have no idea how to read those variables

Type

  M-: system-configuration-options RET

and tell what Emacs displays.  the same with the other one.

> This is emacs 24.5+1-8 as packaged by Debian.
> 
> > What happens if you don't make any of the above customizations?
> 
> Emacs is fine in terms of speed, but it uses the wrong font for
> characters not supported by Fira Sans Mono.
> For example, for ∃ it picks "STIX"

The default fontset was improved in Emacs 25.1, so there you should
have Symbola for this character automatically.

> I acknowledge that it'd be much easier if you could reproduce the
> problem.  That said, there's a patch that Clément said works.  Honestly
> I am a little puzzled why you are so hesitant to apply a patch that you
> wrote, and that reportedly fixes the problem.  Do you expect that patch
> to have negative side-effects?  Of course this is ultimately up to you,
> I am just expressing my surprise here.

I'm hesitant because no one understands why that patch should have any
profound effect on performance.  We arrived at the patch by
selectively reverting a particular commit that was fond by bisection.
But that commit fixed an unrelated bug, so it wasn't done by mistake.