Why hold him back anyway? Who is not interested, let him not listen.
This is an irresponsible approach, because only you are responsible for the result. Remember that the goal of a talk is always a change, some kind of action that you expect from the audience. And if you want to convince people in the audience that they are right and encourage them to take action, then you need them to dive into the topic and listen to you carefully.
At a speech, people’s attention weakens over time, this is an objective feature of human perception. Therefore, it is the speaker’s responsibility to help people stay focused and not lose thought.
How long will they listen to me for sure?
On average, you have 6 minutes at the start of your presentation to engage your audience.
The first 30-40 seconds, when you go on stage and get ready to perform, you are studied with interest. Then comes the most important 5 minutes: the same attention as now will no longer be on the whole performance. At these minutes, the audience catches every word and decides whether to listen to you or get a smartphone, believe your words or take them skeptically.
If you are speaking at a video conference or webinar, you have much less time. Think of how quickly people switch TV channels in search of something interesting. You will have 2-3 minutes to engage your audience on screen.