Human Speech: 39 BPS

Article at Science: https://www.sciencemag.org/news/2019/09/human-speech-may-have-universal-transmission-rate-39-bits-second

Hence the utility of radio modes like PSK31.  It runs at (you guessed it) 31 bits per second, leaving a little bit of processing headroom for other tasks such as trimming filters, tweaking gains, sipping coffee, etc.  There are many other digital modes: RTTY, Olivia, etc. (looking these up on Wikipedia is left as an exercise for the interested reader), and they all have different bit rate variants.  However, almost universally, the most commonly used rate is something in this slightly-less-than-40 BPS range.  Probably not a coincidence.