The bottleneck with SFTP / SCP / SSH is usually the server software - SSH can multiplex streams, so it implements its own TCP-style sliding windows for channel data. Unfortunately OpenSSH and similar server implementations suffer from the exact same problems that TCP did, where the windows don't scale up to modern connection speeds, so the maximum data in-flight quickly gets limited at higher BDPs.

HPN-SSH[1] resolves this but isn't widely deployed.

[1] https://www.psc.edu/hpn-ssh-home/