bug#21028: Slow font rendering in emacs

[Ralf Jung]

Hi,

>> 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.

This takes no noticeable time (<0.1s).

> 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.

I don't think that's true.  The combination of Fira Sans Mono and DejaVu
Mono covers all characters I mentioned, and all characters I ever needed
so far (including even ✓), except for ∖.

> 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.

I noticed that emacs 25.1 is also available in Debian testing, so I'll
upgrade and report on my observations.

Kind regards,
Ralf