Humans are asynchronous receivers, they can pick up words at any time, even if spread out, can combine into a sentence. Data rates are as fast as someone can hear/talk/copy code.
All of the fast digital modes are Synchronous, example the HF digital voice has a lag between someone talking and their voice getting out because of the initial sync code.
On a particular radio Wikipedia page, it claimed these data rates:
AM/FM Synchronous: 12/16 kbit/s
AM/FM Asynchronous: Below 4800 Baud
What aspect of digital technology allows the data rate to be much faster if the transmission is synchronous.