Starting from version 2.0.0-4, nxssh will request X11 forwarding if the connection is encrypted. X11 forwarding will not be used by NX, and the forwarding will still be managed by nxserver. If we don't request X11 forwarding, SSHD will not consider the session interactive and will unilaterally turn on the Nagle algorithm on the remote socket, causing a visible slowdown.
As a consequence, the key added to the nx user on the NX server should not have the 'no-X11-forwarding' option set. If the option is set (for example when connecting to older NX servers) or if the X11 forwarding has been disabled by the system administrator, the NX connection will work unaffected, but the users will be unable to enjoy the best performance when running the session over the encrypted connection.
Please note that this is actually a SSHD problem and there is really little we can do to overcome it.