Pardon my ignorance on the matter, but why does the IPv6 speedtest give much better download results? It's almost 3X as fast.
I think it's because Comcast hands off to Level3 as soon as they can for IPv6. I notice that traceroutes over IPv6 usually have less hops (to the east coast it's sometimes 1/2 the hops of IPv4 to the same server).
[EDIT]
Traceroute to the same VPS in Tampa, FL from my home in Denver, CO:
IPv4:
3 9 ms 9 ms 11 ms xe-9-1-3-0-sur02.arvada.co.denver.comcast.net [162.151.38.209]
4 49 ms 17 ms 10 ms ae-21-0-ar01.aurora.co.denver.comcast.net [68.86.179.213]
5 13 ms 11 ms 11 ms te-0-1-0-4-cr01.chicago.il.ibone.comcast.net [68.86.95.201]
6 34 ms 35 ms 35 ms he-5-15-0-0-cr01.350ecermak.il.ibone.comcast.net [68.86.85.113]
7 49 ms 34 ms 33 ms he-0-2-0-0-cr01.350ecermak.il.ibone.comcast.net [68.86.88.138]
8 33 ms 33 ms 35 ms as3257-pe01.111eighthave.ny.ibone.comcast.net [75.149.229.114]
9 86 ms 87 ms 89 ms 207.88.14.193.ptr.us.xo.net [207.88.14.193]
10 88 ms 88 ms 87 ms vb24.rar3.washington-dc.us.xo.net [207.88.12.34]
11 89 ms 110 ms 87 ms te-3-0-0.rar3.atlanta-ga.us.xo.net [207.88.12.9]
12 95 ms 87 ms 87 ms te-4-0-0.rar3.miami-fl.us.xo.net [207.88.12.6]
13 88 ms 85 ms 85 ms ae0d0.mcr1.tampa-fl.us.xo.net [216.156.0.218]
14 86 ms 87 ms 86 ms 64.220.113.126.ptr.us.xo.net [64.220.113.126]
15 86 ms 87 ms 87 ms v998.core2.esnet.com [208.38.174.26]
16 94 ms 90 ms 92 ms 199.167.31.4
IPv6:
3 9 ms 10 ms 9 ms 2001:558:1c2:ec::1
4 84 ms 16 ms 12 ms ae-21-0-ar01.aurora.co.denver.comcast.net [2001:558:1c0:a3::1]
5 15 ms 15 ms 12 ms 2001:558:0:f5d6::1
6 12 ms 11 ms 11 ms ae14.edge3.Denver1.Level3.net [2001:1900:4:3::269]
7 24 ms 24 ms 24 ms vl-4040.edge2.Dallas1.Level3.net [2001:1900:4:1::3a]
8 52 ms 52 ms 52 ms vl-4044.edge2.Dallas1.Level3.net [2001:1900:4:1::32e]
9 52 ms 52 ms 52 ms vl-11.bar1.Tampa1.Level3.net [2001:1900:4:1::329]
10 56 ms 53 ms 53 ms WIDEOPENWES.bar1.Tampa1.Level3.net [2001:1900:2100::1f4a]
11 54 ms 53 ms 54 ms 2607:f3f0:0:2:2d0:ff:fe9e:2400
12 53 ms 53 ms 53 ms 2602:fff6:f:1::4:4
What's funny is that I'm only paying for 50Mbps and I always get more than that over IPv6 while I rarely hit that for IPv4.