My yardstick for effective public speaking is comedians. Most, I find, speak quite slowly, or at least enunciate very clearly, which makes sense since they want the audience to be able to pick up on all their hard-worked jokes.
This is why I was a bit surprised when I read the suggestion here that rapid-lecturing could serve as an effective technique. The key? One should repeat the point over and over again, so instead of making 2 points in 10 minutes you can make 2*2 points (the same points repeated over) + 3 tangents / 10 minutes, but the retention of the tangents will be low.
The exception that proves the rule: