There are a LOT of factors which can all be combined into: - Tunneling overhead: protocol-41 loses 20 bytes per packet, AYIYA quite a bit more, see the FAQ for the details. - MTU: larger packets means you will be able to send more data per packet and thus might improve speed quite a bit. - QoS on the path between your endpoint and the PoP: anywhere somebody might decide to de-prioritize packets in the networks because they think it is not important it enough. - CPU load, if you are using AYIYA packets are being signed and thus that has some overhead which on slower CPUs might cause a bottleneck. - The path between you then to the PoP over IPv4 and then to the IP you are communicating with over IPv6 might not be fully speed or might be overloaded. - (and lots of other factors which I can't come up with now ;) This is all why native IPv6 is a good thing to have, ask your ISP about it today, and again tomorrow and the day after that too till they get annoyed and get you native IPv6. Every support call costs them on average 75US (according to some survey ;) so go at it!