I find that there's a ~60ms premium for going through the tunnel, relative to straight IPv4 traffic.
IPv4 ping of www.kame.net:
# ping -c 10 126.96.36.199
--- 188.8.131.52 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9002ms
rtt min/avg/max/mdev = 173.042/181.443/192.257/6.115 ms
IPv6 ping of www.kame.net:
ping6 -c 10 2001:200:0:8002:203:47ff:fea5:3085
--- 2001:200:0:8002:203:47ff:fea5:3085 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9000ms
rtt min/avg/max/mdev = 223.853/234.140/244.164/6.195 ms
FWIW, I'm going through the Chicago tunnel server.
Is this performance premium reasonable/typical for tunneled IPv6 traffic?