latency measured on a protocol level from time of request to response.
a bad interface or attached infrastructure can cause latency. but it's not measured against.
i can't re-call snapmirror itself measure latency during transfer. maybe it's avail in some diag command on given moment.
you can test with ping on specific interface and see if you get odd results
network ping -vserver <Cluster name> -lif <lif> -destination <dst filer LIF> -show-detail -packet-size 1300
and in ONTAP 9 you can simulate transfer between nodes and get perf information with "network test-path"