I have a similar issue and from all my testing it points to the replication that causes the performance decrease. I have tried making changes to the replication but cannot improve the performance. I have tested all the 10GB cards using the network test and all tests correct. Eg running iometer replication off /cluster degraded i get RX 1900MBs WR 1800MBs, when cluster normal RX 1250MBs WR 700MBs