bug#21028: Slow font rendering in emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: Ralf Jung <post@ralfj.de>
> Date: Mon, 13 Mar 2017 16:54:01 +0100
> 
> I think I ran into the issue described here as well:  If I am not
> careful with the way I configure my fonts, emacs becomes pretty slow at
> rendering them.  With my old laptop, actually merely navigating the
> cursor large files (like
> <https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp/raw/master/theories/list.v>)
> already was noticeably luggish.

I don't see any sluggish-ness in "emacs -Q" here, FWIW.

Do you have any measurements that would show the slowdown in numbers?
Also, what is your value of system-configuration-options and
system-configuration-features?  And which version of Emacs is that?

> Here's the setup which makes things behave badly:
> 
> (dolist (ft (fontset-list))
>   ; Main font
>   (set-fontset-font ft 'unicode (font-spec :name "Monospace" :size 11.0))
>   ; Fallback font
>   (set-fontset-font ft 'unicode (font-spec :name "DejaVu Sans Mono"
> :size 11.0) nil 'append)
> )
> 
> And here's the one I use instead, which behaves much better:
> 
> (dolist (ft (fontset-list))
>   ; Main font
>   (set-fontset-font ft 'unicode (font-spec :name "Monospace" :size 11.0))
>   ; Fallback font
>   (set-fontset-font ft nil (font-spec :name "DejaVu Sans Mono" :size 11.0))
> )
> 
> (For the record, my "Monospace" font is "Fira Sans Mono".)

What happens if you don't make any of the above customizations?

Also, do you have the Symbola font installed?  If not, can you install
it and see if that makes things better?

> Configuring fonts in emacs to get proper unicode rendering is already
> fairly difficult

Actually, the default fontset is supposed to "just work", so I'd be
interested to hear why you needed to customize that.

> it'd be great if this bug could be fixed

I'm trying.  But it's hard, since for now I cannot even reproduce it.

Thanks.