please check if there is any throttle set in on source or destination controller. if enable try disabling the throttle and check.
7mode: options replication.throttle.enable
Also You can configure a TCP window size for SnapMirror relationships between the 7-Mode volume and the ONTAP volume to improve the SnapMirror transfer throughput so that the replication operations are completed faster.
Ok, well, while a snapmirror is in progress, get a packet trace on the source and another on the destination. Have someone in your networking group who is able to view/understand packet-traces look for anomolies. Additionally, if you open a case with NetApp, they will likely ask for something similar.