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

I noticed whenever I force qt applications to use any GTK theme on Ubuntu 19.10 by adding the line export QT_QPA_PLATFORMTHEME=gtk2 to .profile, the startup time goes from 1-3 seconds to 20+ seconds for vlc and qbittorrent. Why is this and is there anything I can do?

I have ran sudo apt install qt5ct and sudo apt install qt5-style-plugins

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!