[debian] slow startup of GTK3 apps like firefox/seahorse etc as user (runs ok as root)

 This issue happens mainly when you start the X11 session via console, running those apps as root is without any slowdowns.

A solution that works is to source the systemd user session to your X11 environment, eg by entering this line into the .xinitrc

  systemctl --user import-environment DISPLAY XAUTHORITY &