This is very interesting information. Thank you for sharing this. I am very mystified by this that having the tunnel configured on a single client that directly receives the upstream WAN allows full speed through modem mode is very..... odd.

I'm running my 6in4 tunnel on OpenWrt Linksys WRT3200ACM and all my tests were with the Client IPv6 address. I redid them recently to make it as fair as possible. I've had the same issues on DD-WRT, different routers and different Super Hub versions (all in modem mode). Tested two different Virgin Media lines as well, all the same.

I can't pull my network down like that (well I could I have failover, but not with work from home right now), so I can't test this, but very interesting! What it doesn't explain is why 6in4 works fine on other ISPs with the same implementations you'd normally do, so that is why I remain somewhat convinced something on Virgin Media isn't helping the situation. It has been like this for some years now, but there has been reports of higher speeds in the past. Equally, I recently tested IPv6 through Mullvad through Wireguard, so essentially IPv6 over UDP and speeds where very close to native IPv4, so again leads back to specific configuration issue or handling of protocol 41 data packets. Someone did present the argument that 6in4 is single threaded, but if that was the case why can my Linode VPS pull such speeds with the same tests, just different network and routing.

The fact you are also moving to VM business would be interesting to find out your further findings as they use the Hitron router on the VM business side no the Super Hub so I would be very interested if there's any difference, but you'll also be routed differently, so there a few extra variables there.