bug#24918: 25.1; Fonts can make Emacs grind to a halt

[Alan Third]

On Tue, Nov 29, 2016 at 11:29:03AM +0100, Klaus-Dieter Bauer wrote:
> For such font-substituted characters, there are multiple cases where I
> observed delays.
> 
>    1. When the character becomes visible in the current window (small delay
>    if caused by a single input event, but can crash Emacs when
>    scrolling-events, and thus delays, queue up).
>    2. When `point' is moved to the line containing the character, either
>    with mouse or keyboard (somewhat bigger delay).
>    3. When `point' is moved to the character itself, either with mouse or
>    keyboard (extensive delay).
>    4. Whenever there is any change to the Window-layout (e.g. splitting the
>    window, or resizing the frame).

I experience these slow‐downs too on my work (Windows 10) PC.

I found that I could reduce the delays by defining fallback
fonts explicitly using:

    (set-face-attribute 'default nil :font "Droid Sans Mono")
    (set-fontset-font nil 'unicode "DejaVu Sans" nil 'append)
    (set-fontset-font nil 'unicode "Courier New" nil 'append)
    (set-fontset-font nil 'unicode "Symbola" nil 'append)

but it doesn’t get rid of them completely.

Switching to the new optimized Emacs 25 build improved things a bit
too, unsurprisingly.
-- 
Alan Third