Timeout properties are ignored when connecting via a proxy

The problem is that we have a slow sftp server which takes some time to establish connection with. It may take up to 2 minutes to establish a connection, and after that it works fine. When connecting using various ftp-clients we increased timeout property and it worked for us.
However this does not work via the code. When trying to connect via a secured proxy - it seems that both
ConnectionInfo.Timeout and SftpClient.OperationTimeout have no effect and connection is terminated due to a timeout after ~27 seconds.
This seems to be an old issue with at least one other user facing it in 2015
http://stackoverflow.com/questions/34388446/c-sharp-renci-sshnet-sftp-client-connection-behind-proxy-timeout#comment72017745_34388446
For me it the same with Renci.SSH 2014 as well as with the latest version SSH.NET.2016.0.0

Thank you in advance for your consideration of the issue

Beginne damit nachzuverfolgen, wie Proxy-Verbindungen mit ConnectionInfo.Timeout und SftpClient.OperationTimeout umgehen, und reproduziere dann den gemeldeten Fehler mit einem langsamen SFTP-Server. Prüfe, warum die Verbindung trotz der konfigurierten Werte nach etwa 27 Sekunden beendet wird. Als erledigt gilt die Aufgabe, wenn der konfigurierte Timeout beim Verbinden über einen gesicherten Proxy eingehalten wird.