Lately (past couple weeks perhaps) I've been seeing pretty poor performance to Cloudflare-hosted sites through the tunnel at the Seattle POP. Traceroute output:
traceroute to cloudflare.com (2606:4700::6811:b055), 30 hops max, 80 byte packets
1 edgeroute.desu.ne.jp (2001:470:e955:1::1) 0.286 ms 0.267 ms 0.261 ms
2 tunnel398002.tunnel.tserv14.sea1.ipv6.he.net (2001:470:a:c9c::1) 15.072 ms 15.976 ms 16.921 ms
3 10ge1-18.core1.sea1.he.net (2001:470:0:9b::1) 17.837 ms 18.076 ms 18.033 ms
4 * as13335.seattle.megaport.com (2606:a980:0:4::7) 102.445 ms v6-six.as13335.com (2001:504:16::3417) 102.401 ms
5 * * 2400:cb00:28:1024::6ca2:f45f (2400:cb00:28:1024::6ca2:f45f) 102.677 ms
For now I've been rejecting connections to Cloudflare's IPv6 ranges thru my firewall so Happy Eyeballs kicks in to force IPv4. In terms of bandwidth, I'm getting about 8 mbps on speed.cloudflare.com via IPv6, and the expected 300 mbps via IPv4. Other IPv6 destinations seem unaffected; it's only Cloudflare that seems to be slow.
Maybe the peering session between HE and Cloudflare at SIX is congested?