Not an aviation professional, but have professional experience with speaking so as to be understood. Speed of oral delivery varies with language, region, individual, mood, context (including the size of the room, and presence or absence of microphone); in some languages, the unit of expression is the breath-group, not the word (in my inexpert experience, that makes a big difference between Spanish and Italian in their intellgibility to someone with a rudimentary knowledge of the language). But I am surprised that in the extensive training given to ATC people, there seems to be no training, and no standardisation, in speed and style of delivery. Can this really be so?
Has anyone done any research on speed of delivery and the consequences for efficiency in ATC (I imagine a curve, bell-like but with quirks). There are Departments of Applied Linguistics constantly looking for research projects to justify their existence; otherwise, one could assume that a speed that reduces "Say agains" to a very low level would be a good point to aim for.