The Server Monitoring spec explicitly forbids setting a heartbeatFrequencyMS of less than 500ms:

For both multi- and single-threaded drivers, the driver MUST NOT permit users to configure it less than minHeartbeatFrequencyMS (500ms).

Setting a heartbeatFrequencyMS of less than 500ms may only have an observable difference when monitoring 4.4+ servers with streamable ismaster. Previous polling behavior appears to respect the 500ms minimum delay looking at x/mongo/driver/topology/server.go).