Congestion between HE and Cloudflare in Seattle?

[desunet]

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?