I don’t think that as speakers we can make assumptions as to the listening speed of our audiences. They are not homogenous groups. In any audience there will be people that are slow on the uptake and there will be those that want things sped up i.e. chop, chop!
A big challenge for many speakers is gauging how much content to create to fill a given amount of time. A simple formula of 125 words per minute, times the number of minutes that you are allotted to speak, should serve as a guideline.
I think deviating from the 125 to 150 speed limit i.e. words per minute could be a problem. I would find it very difficult to try to slow down to 100 words or so a minute. It would probably be better if I chose simpler, less complex words for my audience to understand and speak at my normal rate. This would have to be with out sounding condescending.
Conversely, speeding my speaking rate up to 175 to 200 words per minute might be okay for short bursts, perhaps to imply excitement on my part as to the content I’m delivering at the time might be okay.
I believe any speech should contain different rates of speaking, that add to the message you are delivering.
If you are delivering humourous content, you could speed up the delivery to build some excitement as you deliver the story and work towards your punchline but you will want to put on the brakes to allow your audience to think about what you said and to laugh. Too many speakers step on their lines, meaning they move on to the next line while the audience is still laughing. Those that are still laughing miss the next point that you want to make.
As for the trial and error approach to determining how fast to speak to the audience that is before you, you have to continually monitor them. Are there audience members starting to doze off? You might want to pick up your pace a little. Are you seeing confusion or frustration in your audience member’s faces? You might want to slow down a little or even pause and ask them if they are finding the content confusing or if they have any questions.
To conclude, while I think it is important to meet your audience’s needs I believe that it is equally important to be true to your self. Speaking out of your normal range, for extended periods of time, would likely be evident and take away from your effectiveness.
Thanks for your question!