Several quarters ago, a student of mine gave a great speech about the ZPD, or “zone of proximal development.” As she explained it to us, every learner (in our case, an audience member) comes into the room with an existing set of knowledge and skills. The zone of proximal development contains the set of knowledge and skills that you as the teacher (speaker) can help them develop. Outside the zone of proximal development are the knowledge and skills that are too advanced for them at this point: while you may be able to help them learn these things in the future, they’re out of reach at this particular time.
Therefore, whenever you’re teaching (speaking), your goal should be to spend most of your time in this ZPD as you can—while it may be good to briefly review what your audience already knows, spending too much time on such review is a waste. Likewise, it’s also a waste of time to spend your time outside of this ZPD—no matter how much you try to explain, the audience won’t get it.
In the end, the key takeaway here is that whenever you are designing a speech, it’s important to: 1) understand where your audience is, 2) understand where it’s possible for your audience to get to, and 3) figure out the best way to move them from A to B.