I've learned that many students lose interest about 2 slides into a presentation. The solution is simple. Don't use presentations.
Spend your time with the students by talking with them. Use the board for sketches, mathematical development, etc. This forces you to go slowly enough that students can copy your work, which is basically how people learn. Since you will be talking with the students (with the lights turned on!), you can turn around see whether they are "getting" the tricky parts, and you can adjust easily. None of this works when you're sleepwalking through a slide deck.
This method also works for computing work. Students can learn a lot by watching somebody live-code a problem. Showing a slide full of code might be helpful to a few students in the class, but most will just turn off. Students can learn a lot by seeing a professor type code, building ideas from the inside out, and catching errors, etc.
So that's the mechanical part. On the logistic part, here's something else I've learned: almost anything can be squeezed into the last 20 minutes of a class, and almost anything can be stretched out to fill the last 20 minutes. Students and teachers are humans, after all. Your friend can tell you a story during an elevator ride, or stretch it out over a few drinks in the pub. Either works.
I dunno if it's the biggest myth about education, but it's gotta be in the top 3: The myth that you can stand in front of someone, speak some words on some topic, and all humans will automatically absorb the words, retain them, correctly make all relevant logical deductions from these words, retain all of this for the next several decades, and no particular effort need to be put into the specific words used and no examination of the audience need be done because humans do all of this automatically for you, 100% reliably.
I do not know where we get this idea. Literally everyone knows from years of personal experience that it is false. I can hardly think of a claim that each and every one of us has had more thoroughly debunked, in the strongest possible manner.
Yet virtually all of us act as if it is true, and indeed, there's even a contingent of people who will attack you if you claim it isn't true.
Lectures can work if you do as described. There's some other ways to make them work too. But there's an awful lot of just standing in front of a bunch of people, flashing some slides, and reading them off, and then, I guess, expecting some sort of miracle to occur because they darned well ought to know what the result of that approach will be from personal experience.
This is the way I taught my math classes at two different universities while I was in grad school. I even used chalk. As I worked through the proof or problem, writing out everything in full meant we could have an interactive flow where if I lost them they could complain. And they did! I would get comments/questions even if I was writing on the board, with my back turned. I don't recall time management ever being an issue, and my practice was to just work through everything beforehand.
It was one of the most satisfying/gratifying experiences of my entire life.
(Dirty secret: I only deeply learned vector calculus/linear algebra when I taught it.)
Spend your time with the students by talking with them. Use the board for sketches, mathematical development, etc. This forces you to go slowly enough that students can copy your work, which is basically how people learn. Since you will be talking with the students (with the lights turned on!), you can turn around see whether they are "getting" the tricky parts, and you can adjust easily. None of this works when you're sleepwalking through a slide deck.
This method also works for computing work. Students can learn a lot by watching somebody live-code a problem. Showing a slide full of code might be helpful to a few students in the class, but most will just turn off. Students can learn a lot by seeing a professor type code, building ideas from the inside out, and catching errors, etc.
So that's the mechanical part. On the logistic part, here's something else I've learned: almost anything can be squeezed into the last 20 minutes of a class, and almost anything can be stretched out to fill the last 20 minutes. Students and teachers are humans, after all. Your friend can tell you a story during an elevator ride, or stretch it out over a few drinks in the pub. Either works.