Re: Run `M-x list-input-methods` and then scroll down the screen in input method panel using the mouse or arrow key will cause Emacs unresponsive for about 1 minite.

[Eli Zaretskii]

> Date: Sat, 19 Jun 2021 13:55:51 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: emacs-devel@gnu.org
> 
> The fast version is an optimized build, the slow one is unoptimized.
> That explains at least part of the slowness.  The fast version also
> doesn't include any text-shaping engine, which might speed up Emacs,
> but prevents it from correctly displaying complex scripts.
> 
> What are the Emacs versions of the two builds?

Sorry, I see now that its Emacs 26.3 and 28.

And I guess the Emacs 28 build also used the default optimization
options, unlike what I said before.

So the only difference I see is the text-shaping engine that is
missing in the "fast" version.  I don't think I understand why that
should have such a profound effect on the speed of finding fonts,
sorry.