The Server Discovery And Monitoring Spec says a driver must wait at least minHeartbeatFrequencyMS (500ms, not configurable) between ismaster calls no matter what:

https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#minheartbeatfrequencyms

If the C Driver is configured with a heartbeatFrequencyMS shorter than that, it can call ismaster more frequently.

I think this MIN should be a MAX:

https://github.com/mongodb/mongo-c-driver/blob/5972a47c1c8ca7bf17056492573579baeb2c9c40/src/mongoc/mongoc-topology.c#L629-629