I appreciate the attempt at making sense of conversational interfaces, but I don't think natural language as a "data transfer mechanism" is a productive way of doing it.
Natural language isn't best described as data transfer. It's primarily a mechanism for collaboration and negotiation. A speech act isn't transferring data, it's an action with intent. Viewed as such the key metrics are not speed and loss, but successful coordination.
This is a case where a computer science stance isn't fruitful, and it's best to look through a linguistics lens.