IPv6 uneven by region, network; IPv4 still has higher connection success rate

Measurements comparing round trip time of IPv4 and IPv6 paths between the same two dual stack endpoints show the two protocols are roughly equivalent on average, but with some significant outliers. At a finer level of detail, there are discernible differences between geographic regions. IPv6 appears to be faster on average in Europe, Africa and Oceania, while IPv4 is faster in Asia, and the Americas. At a further level of detail these differences between IPv4 and IPv6 extend to individual access networks where some networks appear to handle IPv6 packets in an entirely different manner to IPv4 packets.  Meanwhile, although the TCP connection performance is roughly equivalent once the connection is established, the probability of establishing the connection is not the same, with a failure rate for IPv4 connections at approximately 0.2% of all connection attempts, and the connection failure for unicast IPv6 eight times higher, at 1.6%.