It looks like human speech may have a universal transmission rate of 39 bit per second.

Italians are some of the fastest speakers on the planet, chattering at up to nine syllables per second. Many Germans, on the other hand, are slow enunciators, delivering five to six syllables in the same amount of time. Yet in any given minute, Italians and Germans convey roughly the same amount of information […]: they tend to transmit information at about the same rate: 39 bits per second.

This is extremely interesting, as it applies to written languages, where information density per syllable was investigated, as well as spoken languages. The conclusion is also fascinating:

But the “why” is another question entirely. Pellegrino and his colleagues suspect that the answer has everything to do with the limits imposed by […] how much information our brains can take in—or produce—at any one time. […] De Boer agrees that our brains are the bottleneck. But, he says, instead of being limited by how quickly we can process information by listening, we’re likely limited by how quickly we can gather our thoughts. That’s because, he says, the average person can listen to audio recordings sped up to about 120%—and still have no problems with comprehension. “It really seems that the bottleneck is in putting the ideas together.”