There's a defaultLocalThreshold constant defined (to 15 ms) but it's completely unreferenced. It seems like the intention was to make Client.localThreshold default to defaultLocalThreshold, possibly in newClient.
Confusingly, there is one usage of description.LatencySelector(15 * time.Millisecond), in Operation.selectServer, but my read of that code is that it would almost never come into play, since a selector will almost always be specified. Either way, it seems like that should use defaultLocalThreshold, rather than hardcoding 15 * time.Millisecond.
I suspect all of this means that localThresholdMS ends up defaulting to 0 ms.