That said, I recently had to disable IPv6 on my parent's ISP for bandwidth reasons. They live in France and are served by Free[1], who enabled IPv6 a few years ago. This christmas, I was surprised by their terrible bandwidth, which was worse in the middle of Paris than something in a village in the alps! I ultimitely traced it to the IPv6 option, though I didn't do deeper tracing to find out why.
I suppose it's because many legacy backbone routers don't handle IPv6 at their top capacity, as they have HW to accelerate IPv4 but not IPv6. See for example the question "What is the difference between hardware and software IPv6 acceleration?" in this FAQ at Cisco: [2].
A lot of the problem is probably Free and its policies. The way Free bullied, badgered and French-protectionism'ed its way into the various peering agreements (or lack thereof) could explain why v6 connectivity is worse.
That said, I recently had to disable IPv6 on my parent's ISP for bandwidth reasons. They live in France and are served by Free[1], who enabled IPv6 a few years ago. This christmas, I was surprised by their terrible bandwidth, which was worse in the middle of Paris than something in a village in the alps! I ultimitely traced it to the IPv6 option, though I didn't do deeper tracing to find out why.
I suppose it's because many legacy backbone routers don't handle IPv6 at their top capacity, as they have HW to accelerate IPv4 but not IPv6. See for example the question "What is the difference between hardware and software IPv6 acceleration?" in this FAQ at Cisco: [2].
[1]: http://www.free.fr [2]: http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/p...