As your samba speed is roughly half your measured network speed (iperf3) I wandered if the client in use was using libsmbclient, i.e. a linux desktop client with GUI administered samba client connection to Rockstor’s samba server.
See the following thread if that is the case:
That’s all that springs to mind currently. Not sure is this is still currently relevant but linking just in case as easy to test given the info in that thread.