speed of replication depends on
1. amount of data to be transferred
you can check it on your source with command
::>volume snapshot show-delta
and compare the difference between the last transferred snapshot with active file system
This may give a rough expectation of how much data is going to be transferred per volume
2. link bandwidth between sites.
there is a workaround to establish a baseline copy between two sites with a poor link using an external USB drive
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Protection_and_Security/SnapMirror/How_to__consider_for_using_Smtape_to_seed_a_SnapMirror_relati...