@JP2222 Thanks for making the discovery about disconnecting the network!
I noticed the IDE accesses api-builder.arduino.cc on startup (thank you wireshark) so I blocked access to this host. I added an entry to /etc/hosts like this to blacklist the host.
127.0.0.1 api-builder.arduino.cc
Now the IDE starts up with the serial ports listed immediately without having to disconnect the network cable.
Perhaps something has changed on api-builder.arduino.cc.