Why does forcing Qt applications to use GTK theme makes those apps startup slowly?

I did some testing as I had some machines with the problem and others without it. I have narrowed it down to this:

sudo apt install appmenu-registrar vala-panel-appmenu-common

With those both installed (and the recommends from appmenu-registrar), after reboot your qt5 apps should launch without delay, even with QT_QPA_PLATFORMTHEME=gtk2 in your ~/.profile or /etc/environment So GTK theme looks good plus no delay in launching qt apps!