this will rebuild the system font cache in /var/cache/fontconfig
If you then delete the user's cache in ~/.cache/fontconfig and re-log, there is no delay... and the user's font cache will NOT be rebuilt.
After a delayed login, user's cache and the system cache appear to contain the same files.
So it looks as if something thinks the system cache is out of date and wants to rebuild it. Whatever it is only has user permissions, so rebuilds the font cache in the user's .cache
Another piece of the puzzle: if you run:
sudo fc-cache -fv
this will rebuild the system font cache in /var/cache/ fontconfig
If you then delete the user's cache in ~/.cache/fontconfig and re-log, there is no delay... and the user's font cache will NOT be rebuilt.
After a delayed login, user's cache and the system cache appear to contain the same files.
So it looks as if something thinks the system cache is out of date and wants to rebuild it. Whatever it is only has user permissions, so rebuilds the font cache in the user's .cache