slow echo test

create the first file: /etc/bigbluebutton/nginx/bigbluebutton_sip_addr_map.conf with both my servers IPv6 and IPv4 addresses in it
change in the file /etc/bigbluebutton/nginx/sip.nginx the proxy_pass line (replace IPv4 adress with the variable $freeswitch_addr)
edit /opt/freeswitch/conf/sip_profiles/external-ipv6.xml where I only had to uncomment the enable-3pcc line.
After bbb-conf --restart the time it takes until echo test actually connects is MUCH shorter now. Sometimes it took 30 seconds before, now between 1 and 5 seconds (not much load on the server when testing).

fix the STUN server in /opt/freeswitch/conf/vars.xml and in /usr/share/bbb-web/WEB-INF/classes/spring/turn-stun-servers.xml.
If possible setup and use your own STUN server. Do not use stun.freeswitch.org.
The delay should go down to 1-2s, or to about 5-7s, depending on the clients.

Finally I desperately tried a sudo bbb-conf --setip bbb.mydomain.com And that did the trick! Hope this will help people.

Just launch FireFox, open about:config, and search for media.peerconnection.ice.relay_only. Set this value to true.