Hi Chris. Thanks for taking the time to respond, I'm glad this provided some insight! I have also used Hurricane Electric for years and suffered from the mentioned performance issues. It's 100% a Virgin Media problem. I've had this happen on multiple VM connections and different variants of the Super Hub in modem mode, the common factor is always Virgin Media.

Right now based on what I and others have found, encapsulating 6in4 over UDP seems to be the only realistic way to avoid whatever VM are doing to proto 41. Testing IPv6 with Wireguard, OpenVPN and also someone recently tested Cloudflare Warp with much better performance.

If the tunnel doesn't terminate on a VM IP, you're probably OK, so having a tunnel on a VPS which is then connected via OpenVPN or Wireguard passing down a routed prefix seems like the only alternative, but potentially more expensive workaround, with an extra layer to deal with.

Ah that will be the day when Virgin Media and it's IPv6 plans actually happen, although if it's the DS-Lite config they tested in 2018, no thanks! They'll never take modem mode away from me unless it's from my cold dead hands.