There are patches for openssh when using it over LFNs: https://psc.edu/research/networking/hpn-ssh/ (which seems to be in a sad state currently). These patches might interact poorly with other implementations (e.g., it didn't interoperate with paramiko (2 years ago)).

Also, make sure TCP window scaling is working. I was making transfers through a F5 Big-IP which was running a profile that disabled it.