Users on Centurylink Fiber can reach us in 4ms over IPv6 (despite it not being true IPv6) versus 45ms to 60ms with IPv4. DSL is similiar but not quite as notable, more like 30ms of saved latency.
The root issue is Centurylink has no peering with Hurricane Electric on IPv4, and ditto for peering with numerous other providers, versus on IPv6 they have a much more open peering policy. This seems to be common among a number of ISPs, though as time goes on this issue will hopefully ease.
More like the end user gets routed through bespoke connections hundreds of miles away due to ISPs like Centurylink not openly peering on IPv4 at Internet Exchanges, whereas on IPv6 they seem to have local routes to Hurricane Electric and many others on the West Coast, thus no need for traffic to go back east!
The root issue is Centurylink has no peering with Hurricane Electric on IPv4, and ditto for peering with numerous other providers, versus on IPv6 they have a much more open peering policy. This seems to be common among a number of ISPs, though as time goes on this issue will hopefully ease.