Googling, I found some similar bug reports:

https://bbs.archlinux.org/viewtopic.php?id=243737
https://forum.level1techs.com/t/gtk-applications-are-super-slow-at-startup/166540

The Arch discussion is very similar, as we are both using i3 launched
with startx.

The Arch discussion pointed to their wiki page on xinit, which
includes this note:

------
Note: At the very least, ensure that the last if block in
/etc/X11/xinit/xinitrc is present in your ~/.xinitrc file to ensure that
the scripts in /etc/X11/xinit/xinitrc.d are sourced.
------

I did that, and it fixed the problem.

The file /etc/X11/xinit/xinitrc on my Debian system merely sources
/etc/X11/Xsession, which is attached for your reference.

It does a lot of things and references other files... I have no idea
what actually fixed my problem.