I think the catch is that human detection of pauses is far more fraught with difficulty than you'd imagine. Overly emphasized pauses are easy to discern, but quite a lot of speech pulls words far closer together than you would think. And this is just in english. Not all languages work with emphasis syllables and such.
Exactly, and the difference of the length of pauses between 'syntactic' words (peanut, butter) and between compounded words (peanut-butter) will vary based on speaker and moreover, things like focus/emphasis can cause lengthening of pauses even for the latter case. ("No, Alexa, I'm not shopping for Whistler's Mother; I need to buy PEANUT BUTTER.")
