I really like the idea of being very clear on the key concepts of the talk. Although an intermediate or higher talk might expect some knowledge of the audience, it makes sense to provide a refresher on the concepts you see as the building blocks that you’re building on later in the talk. That way you’re including the 20% who might not be familiar with those concepts, but you’re also “priming” people’s minds, making it easier for even the 80% of the audience to follow along.

I’d say the trick is knowing what is relevant and what isn’t. Preparing for a talk you’re likely to pick up a lot of information that isn’t strictly necessary for the audience to understand the talk. Putting in the effort to explain those parts would just be confusing. I’m not suggesting you do that though

So if you are talking about specific topic such as implementation of some protocol, it’s always good idea to go two steps back and describe what the problem is, what are the solutions out there. Explain that there is this protocol that handles these cases and it’s good because of X, and that your library implements it. And then that something uses this protocol to do Y, better than say using HTTP and there are real life implementations and give examples.

I have seen quite a few talks where they are focusing on their library / implementation of something, without explaining much of what the actual problem this library solves, what can it be used for etc.

With my GraphQL talk I did last year couple times I put like 1/3 of the talk to describe what it is, how was created, what it is used for ect. before I even started talking Elixir. I did a trial run without, and people were confused like they knew sort of what GraphQL is but not quite.

And 2nd thing, if you do the above and spend 15 mins explaining what the original problem is, and there are people in the room that obviously know it well, they will get bored. So, I figured that the first 15 mins where I explain the problem is also good place to insert some jokes.

I got really good feedback after this talk, people really just complained about slides (and rightfully so). But I’m going to use this format one as a template for my future talks (when/if I find some time…). https://www.youtube.com/watch?v=K_9IXTUwVL4

I agree with you. A couple of points: even if a lost of the talk is going over things the audience mostly already knows, that means a lot of it is new to them too.
If everything in a 40 min talk was unfamiliar to me as an attendee I would leave there trying to gather it all.
Also, there is a broader audience that will watch it online. Its worth considering that audience too.
The risk of alienating the part of your audience that is new to the ecosystem is greater than patronizing the ones that are experts.