Starting programs when no internet connection is available

Hello,

I'm encountering a strange issue with my Fedora 27 Gnome installation.
I'm sometimes connected to a WiFi network without a working Internet
connection. When that's the case, some programs like Firefox,
Thunderbird or even LibreOffice take really long to start. As soon as I
disable the connection to that particular network (e.g. by enabling
Airplane Mode), everything works smooth again.

Any thoughts why that might be?

Regards,
Nils.