There’s a lovely experiment from 1929 (Jersild) that compares the effectiveness of common speaking techniques--(or "vividness devices" (67)--on the retention of meaningful information including slowing down and repetition.
Contrary to common habit, speaking slowly is ineffective, because “no active review of a statement can take place until the last word has been spoken, since not until then will the content of the statement be known.” (68) That is, people process and understand your point holistically, once they have the entire idea in mind, not in the order that words are spoken. People also interpret meaningful sections of sentences before modifiers, making instructions in the form of a negative such as “don’t skip your veges” less effective than “eat your veges”, or telling a child, “be careful crossing the road” less effective than “stop”. Encapsulating an idea swiftly and succinctly improves the odds of successful transmission.
However, slowing down through strategic use of pauses between ideas can improve memory retention because it facilitates active rehearsal and consolidation. Rushing from one idea to the next stops listener reflection and review—increasing the odds of information going ‘in one ear and out the other’.
Nonetheless, speaker repetition of ideas can be effective, just so long as there is a lapse of time between repetitions. Immediately repeating what you have just said, hinders learning because it prevents active rehearsal, or as Jersild so beautifully phrases it, “the revival of an impression after the elapse of an interval of time is more profitable than the opportunity to review the item immediately after its initial reception.” (68) In summary: be concise, pause, delay and revise
References:
Jersild, A. (1929). Primacy, Recency, Frequency and Vividness. Journal of Experimental Psychology, 12(1), 58-70.