Why does the transfer speed go down when replication is configured?
When the replication source data is updated, the data is transferred to the replication target.
This data transfer causes a load increase to the TeraStation, which results in lower transfer speed.
For your reference, especially the following cases make the transfer speed low.
A simultaneous access to the replication source.
A multiple replication target set-up corresponding a single replication source.
A continuous modification of a huge amount of files on the replication source for a long time.
A modification of files on the replication source through AFP connection on Mac OS.