SFTP issue with high bandwidth high latency connections

All of that is configured into the rclone config in advance. Some notes on the setup for Duplicati:

Rclone backend documentation
Rclone local repository

and if you want code:

https://github.com/duplicati/duplicati/tree/master/Duplicati/Library/Backend/Rclone

Note that it implements IBackend not IStreamingBackend, so UI might be a little bit less featured.

If you feel current Duplicati manual could use clarification, pull requests can be submitted to it.

Actually, pull requests can be submitted to Duplicati too, and volunteers are very much sought.