There's probably no setting you can change that will help at the moment.
We are going to be replacing the FTP and SFTP code with a new library which should resolve these issues (and some others). That's the first big piece of work we have planned for Opus 13, once the dust settles on the initial release, and will be included in a free update.
(It's a problem for me as well now I have much faster upload speed on my internet.)