This occurs in openSUSE Leap 15.3 (64 bit).
Gtk2 applications start with a 25 seconds delay. That is, it takes exactly 25 seconds the application window to appear.
This behavior has been reported previously in other distros, eg:
In 20.04.1 all gtk2 apps starts 25 seconds https://bugs.launchpad.net/ubuntu/+bug/1891245
I installed appmenu-gtk2-module and libcanberra-gtk-module but it did not solve the problem.
How can I fix this in Leap 15.3?