This number represents the rate of information flow . Some words are longer than others or have more sounds in them so we make tiny adjustments to our rate of speaking in order to maintain this constant rate of information flow. Most American English speakers speak at a rate of 5-7 syllables per second. We speed up our rate of speaking on longer words with more syllables or sounds in them and slow down our rate on short words of one syllable with fewer sounds in them. The third aspect of fluency that we have an intuitive sense about is the effort of the speaker.
Fluent speech looks easy and effortless in two ways.
First, fluent speakers seem to put very little muscular or physical effort into the act of talking. Talking requires very little physical energy from us compared to the amount of energy required to do other common things like walking and writing. Second, fluent speakers seem to put very little mental effort into the act of talking. Rarely do we need to think much about what we do when we talk and how our mouths move. We put most of our mental energy into composing our message and listening to our partner's message.