How Long Can Your Brain Focus?

I remember back then in school, when we had a boring lecture that lasted over 4 hours.

The Lecturer came into the class and starting treating a topic I was obviously interested in.

But his explanation of the topic and the way he presented it was so boring a lot of us got tired after the first 20 minutes.

If that same topic was treated by a very lively lecturer, I'm sure a couple of us would have been able to hold on for a little while longer.

But due to the boring explanation, we got tired almost immediately.

According to some research, the brain loses focus after about 20 minutes of dwelling on a particular subject matter.

And this is why, for good speakers, the first 20 minutes should be used to capture the heart of the audience.
Once the first 20-30 minutes is used wisely to capture their hearts, it becomes easier for the audience to choose to re-focus on the subject matter being treated.

The point is, hit the nail on the head and then start buttressing your points afterwards.

Source
This would keep the audience intact and focused on you, the speaker.

It irks me when I attend events and the speakers are all over gallivanting and wasting time for the first 20 - 30 minutes.

I immediately feel lost. For that moment, the speaker has lost me (and of course many others as well).... All we think of is leaving the venue/event almost immediately.