bug#21028: Slow font rendering in emacs

[Eli Zaretskii]

> Cc: 21028@debbugs.gnu.org
> From: Ralf Jung <post@ralfj.de>
> Date: Tue, 14 Mar 2017 16:57:33 +0100
> 
> Okay, so what I do as actually quite simple: I open the aforementioned
> file in emacs, and then I use the scroll wheel on my mouse to scroll
> around quickly (move wheel up fast - move wheel down fast - repeat).
> When I do this with the "good" setup, text keeps flying over my screen
> and everything reacts instantaneously.  When I do this with the "bad"
> setup, nothing moves until I stop turning the scroll wheel and wait a
> little more, where the waiting time is roughly half the time I spent
> scrolling quickly.

In the "bad" setup, if you press C-n just once, how much time does it
take for Emacs to react?  One second, 10 sec, more?  Same question
regarding C-v.

> >> 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.
> 
> But I don't want Symbola for this character.  Symbola is not monospace,
> so it's not going to look right in a document that otherwise uses
> monospace characters.

I see.  Well, I don't think there are many fonts out there with good
coverage of symbols, punctuation, and other special characters, that
are monospaced.  Certainly not free fonts.  So if you want these
symbols to be displayed monospaced as much as possible, you have 2
alternatives:

  1) find a monospaced font with good coverage of both Latin and
     symbol/punctuation blocks and use it as the default font; Emacs
     will try to use the default font for any symbol/punctuation
     characters that font supports, before trying other fonts

  2) set up a very detailed fontset with explicit ranges of codepoints
     allotted to each font that supports the respective characters
     well and whose looks on display you like

Note that the above is probably relevant to Emacs 25.1 and later (the
first item definitely), so I suggest to upgrade, because this stuff
should work much better in Emacs 25, and some of your problems might
just go away without any need for customization.