Not sure. The only thing that comes to mind are clients that prefer ipv6 over ipv4. In Linux this is defined or defaulted via /etc/gai.conf. If the file is a

Not sure. The only thing that comes to mind are clients that prefer ipv6 over ipv4. In Linux this is defined or defaulted via /etc/gai.conf. If the file is absent ipv6 will be preferred by some DNS libraries and applications. Those clients will try ipv6 DNS lookups and if that does not exist will retry with ipv4. That adds the delay of RTT of the DNS server combined with cache or lack thereof.