Comment 5 for bug 1822370

Interesting and maybe related

https://communities.vmware.com/thread/590825
https://github.com/vmware/open-vm-tools/issues/287
https://bugzilla.redhat.com/show_bug.cgi?id=1624437

The TL;DR of those is that VMWare would have issues with the AF21 QoS flag.
Does your client run in VMWare by chance?

Even if not it might be another part of the network setup between your client and server that reacts to the same change.

Please give the:
  $ ssh -o IPQoS=throughput user@host
a check (older default) if that resolves the issue.

I checked for the reason of this default change and found:
I found that the man page has this update about the defaults between cosmic/disco.
The upstream change is:
  https://anongit.mindrot.org/openssh.git/commit/?id=5ee8448ad7c306f05a9f56769f95336a8269f379

There as no follow on change to that yet as far as I can tell from git.

I'd appreciate if you could do the testing with the QoS options (which can also be set in /etc/ssh/ssh_config if you want to make them permanent. Maybe the world isn't ready for the new defaults yet and we might have to hold them back a release?