Thanks to all people pointing me towards a DNS issue. It appeared that two IPv6 recursive DNS servers were advertised (notably over RFC 7106 RA) and one of them was not responding. Causing ultra-slow FQDN resolution for the dual-stack / IPv6-only Androids...
Problem is now fixed and it seems that the slowness issue is solved. A wonderful list :-) which gained some more subscribers from that ISP. Kind regards -éric >
