bug#21028: Slow font rendering in emacs

[Ralf Jung]

Hi all,

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.  My new laptop is fast enough to make
that not noticeable, but when I actually process the file (it's a file
to be run by the Coq proof assistant, I am using the ProofGeneral emacs
plugin for that), then it moves through the file much slower when I set
up my fonts in the "wrong" way.

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".)

Configuring fonts in emacs to get proper unicode rendering is already
fairly difficult; it'd be great if this bug could be fixed so that one
doesn't have to care both about performance and about selecting the
right glyph for the right character.

Kind regards,
Ralf