What is the Just Noticeable Difference for tempo in speech?

Tempo (speaking rate) varies both between and within speakers. Such variations in tempo are easily noticeable. But what is the just noticeable difference for tempo in speech? As a first approximation, between-speaker tempo variation is quantified in a sample of similar interviews with 80 speakers of Dutch. Second, the JND is assessed using a somewhat unconventional method, viz. detection of tempo drift. This results in a JND of 15%; it is argued that this value is upwardly biased. Third, the JND is assessed using a conventional same~different pairwise comparison, yielding a JND of about 10%. Tempo variations between and within speakers typically exceed this JND, and are therefore potentially important in speech communication.