If the file being downloaded is very large the number of unconfirmed reads is effectively unbounded. For example, when the file is 1.3GB, the number of unconfirmed reads is set to 39K. This results in the client submitting 39K calls to the server, updating the progress to look like the download is done, and then failing when some of the calls don't get responses in time. As far as I can tell, there is no way for the server to push back against these requests. As such, Cyberduck should limit the number of unconfirmed reads to something more reasonable, for example 1K.

FYI, the command like SFTP client supports 64 unconfirmed requests by default:

~ ❯❯❯ sftp -R                                                                                                                
 -- number of outstanding requests (default: 64)

Capping this at a reasonable number like 64 also means that when you click "stop" that it stops very quickly rather than taking a while to drain all of the unconfirmed reads.