It took one day to lower the priority of IPv6 on one Ubuntu servers at Hetzner.de, as the IPv6 network was unstable, some hosts were periodically unavailable by IPv6, delays appeared, etc.
To decrease the priority, just open the /etc/gai.conf file in the text editor (for example, nano, in which Ctrl+X to exit, and y/n to save or cancel changes):
nano /etc/gai.conf
Find the line there:
#precedence ::ffff:0:0/96 100
And uncomment it or add:
precedence ::ffff:0:0/96 100
Done, see also my article:
Disabling IPv6 on Linux