[Terr_]

Yeah, the sender-side is probably the main bottleneck, just consider how often people speak filler phonemes like "uhm", it's so common that you might not notice unless you're looking for them. They are basically placeholders into the data-stream, to indicate that it isn't over yet but there's a delay producing the next item.

In contrast, consider a listener who is equally focused and invested as the producer: They don't often indicate that their own buffer is full or request a repeat. While you may say "hold up" or "run that by me again", it's usually for reasons other than word-rate. (For example, to prompt the producer to try another encoding, to express disbelief or contempt, etc.)