The time between heartbeat pings is hardcoded (https://github.com/mongodb/mongo/blob/master/src/mongo/db/repl/heartbeat.cpp#L373) to be 2000 milliseconds. This amounts to quite a lot of traffic for a backup node on an expensive link (it works out at about 3 GB / month / node if there are only two nodes and communication is over a gvpe network).

It would be useful if it was possible to configure the heartbeat ping time to alternative values.