Far as I know, the poor performance in SFTP setups is not because of the protocol, but OpenSSH. I've noticed this several times with dead slow SSH sessions when doing sshfs. In the paper, the SFTP protocol is better to deal with multiple operations and it works with packets (vs FTP which is text based) but it is limited by other software in the stack.
FTP and SFTP are both old solutions, and as I mentioned before,
OpenStack suits better here. That is basically a self-hosted S3 so it is built for what Chevereto needs to do with external storage. It supports multiple file operations, uses all the available speed, secure, etc.