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[JenkinsNET]

Like many people I have been suffering with this problem (very low throughput over my tunnel compared to native IPv4) for years. I too am on Virgin Media (currently 350 Mbit service). I can easily saturate my connection using IPv4 but a similar test over the tunnel rarely exceeds 10-20 Mbit/s  Many people have suggested that maybe VM are somehow traffic shaping or otherwise 'managing' protocol 41.

Now, my tunnel terminates in my router (Apple AirPort Extreme [802.11ac tower model] with latest firmware) which seems to do a good job... or does it?

As an experiment I connected one of my Macs directly to my cable modem (which runs in modem mode) and created a tunnel in macOS. This gives me a single /128 endpoint on the Mac (the tunnel client address, so no routing occurring on my end in this simple test). Now I am easily able to saturate my connection over both IPv4 and IPv6!

If this issue was simply due to VM slowing/managing/filtering protocol 41 then I should have seen the same issue for this test, but I don't. It seems to me on the basis of this evidence that the issue is either in the AirPort Extreme (poor tunnel implementation and/or poor IPv6 routing implementation) - possible but seems unlikely it could be *this* bad? Or there is some more subtle issue at work.

[cholzhauer]

It seems like there's a problem with the Extreme...can you prove the point again by connecting it to a Linux/Windows machine and re-running your test?