Dear Pepe,
thank you for you reply.
[quote=“Pepe Le Pew”]Try: [Linux]ROOT6 very slow startup
and/or: https://root-forum.cern.ch/t/root-slow-at-startup-when-not-connected-to-the-internet/10147/24[/quote]
The second link worked for me! 
In particular the command suggested in the last post of the thread:
sudo scutil --set HostName name_of_my_mac.local
I hope this doesn’t screw up the start up when I am working in the network back at work !
Riccardo