On 2016-12-19 11:58, Eli Zaretskii wrote:
> Also, do you see the slow-down when just displaying characters from
> this font, or only in the prettify-symbols mode?

Displaying symbols from that font is enough.  Given this, here's an updated recipe:

    /build/emacs/25/src/emacs -Q -l 21028.fixed.el 21028.fixed

Things are actually smoother using :family and :registry, but they are still unusable.  The problem is especially visible with vertical scrolling.

Here is a concrete experiment:

# With :name
$ time /build/emacs/25/src/emacs -Q -l 21028.el 21028.fixed --eval "(run-with-idle-timer 0 0 (lambda () (forward-line 25) (redisplay) (run-with-idle-timer 0 0 'kill-emacs)))"

real	0m3.500s
user	0m1.400s
sys	0m0.824s


# With :family and :registry
$ time /build/emacs/25/src/emacs -Q -l 21028.fixed.el 21028.fixed --eval "(run-with-idle-timer 0 0 (lambda () (forward-line 25) (redisplay) (run-with-idle-timer 0 0 'kill-emacs)))"

real	0m1.487s
user	0m0.752s
sys	0m0.188s


# With your patch from last month applied and the original one reverted:
$ time /build/emacs/master/src/emacs -Q -l 21028.fixed.el 21028.fixed --eval "(run-with-idle-timer 0 0 (lambda () (forward-line 25) (redisplay) (run-with-idle-timer 0 0 'kill-emacs)))"

real	0m0.486s
user	0m0.236s
sys	0m0.028s

Cheers,
Clément.