bug#26402: 25.2; Font rendering extremely slow on any non-default font

[Eli Zaretskii]

> Cc: 26402@debbugs.gnu.org
> From: Gennady Uraltsev <gennady.uraltsev@gmail.com>
> Date: Sun, 9 Apr 2017 00:33:23 +0200
> 
> I recompiled from latest git of today so emacs decalres version 26.0.50
> I tried recompiling without your patch and also with your patch. No
> apparent improvement
> 
> I tried running both (text-scale-adjust INC) and buffer-set-face
> both with and without (setq inhibit-compacting-font-caches t)

Then I think this is a problem we never saw before.  Unfortunately,
since I'm unable to reproduce this on my machine, I cannot debug it
further.  One last idea I have is to upgrade all the relevant
libraries to their latest versions.

> The profiler profiles are attached.

The profiles clearly tell the problem is related to font metrics, but
they don't tell what exactly causes the slow-down or why.  Sorry.