Really slow over IPv6

[#1]

Tried downloading http://mirrors.vooservers.com/centos/6.5/isos/x86_64/CentOS-6.5-x86_64-netinstall.iso
I was getting 11KB/s, switched to IPv4 and getting 3.71MB/s
I don't think it's an issue on their end as I get 4+MB/s for that download over IPv6 from a server in France.

[#2]

I just downloaded that file via my IPv6 Hurricane Electric 6in4 tunnel, and it downloaded at near full line speed.
My connection is 20CN ADSL1 with a full 7.15Mbps profile, and the above iso file downloaded at 6.92Mbps. 
It doesn't get any faster than that on my connection 

[#3]

Very strange behaviour:

Quote

--2014-05-07 18:23:24--  http://mirrors.vooservers.com/centos/6.5/os/i386/Packages/kdegames-4.3.4-5.el6.i686.rpm Resolving mirrors.vooservers.com... 2a00:1c10:3:634::1000:0:1 Connecting to mirrors.vooservers.com|2a00:1c10:3:634::1000:0:1|:80... connected. 9.95KB/s  eta 68m 43s

Quote

--2014-05-07 18:23:30--  http://mirrors.vooservers.com/centos/6.5/os/i386/Packages/kdegames-4.3.4-5.el6.i686.rpm Resolving mirrors.vooservers.com... 2a00:1c10:3:634::1000:0:1 Connecting to mirrors.vooservers.com|2a00:1c10:3:634::1000:0:1|:80... connected. 9,015,152  10.7MB/s