bug#21028: Performance regression in revision af1a69f4d17a482c359d98c00ef86fac835b5fac (Apr 2014).

[Eli Zaretskii]

> Date: Fri, 28 Oct 2016 17:33:43 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 21028@debbugs.gnu.org
> 
> Thanks.  So, like I said: let's revisit this in a month or two,
> assuming no other problems surface.

Given my latest discoveries related to bug#24953, one other thing I'd
suggest to try, with an unpatched build from the current emacs-25
branch, is, instead of this:

  (set-fontset-font t 'unicode (font-spec :name "Arial") nil 'append)

to use this:

  (set-fontset-font t 'unicode
                    (font-spec :family "Arial" :registry "iso10646-1")
                    nil 'append)

IOW, explicitly tell that the font's registry is iso10646-1.  Does
this per chance resolve the performance problem?