Assuming a constant stream of operations totalling 50MB/s data (eg. insertions) to a replica-set, any Secondary that has more than ~40ms network latency (~80ms ping) to its next nearest member will be unable to keep up regardless of the network bandwidth between them.

If the ping time between any two members of a replica set is known, the upper limit for replication throughput in MB/s between those two members can be calculated with the following equation:

4 * 1000 / ping_time_in_ms

The limit arises because the tailable cursor retrieves at max 4MB per roundtrip but the request/reply is serial. Thus, the longer a roundtrip (ping) takes, the lower the throughput.