Most people lose focus during lectures. Our brains are not designed for it. That is why we call it 'death by PowerPoint'. At the same time, labs without explanation are often empty steps performed to get a grade. Labs and Lecture really belong together. Keeping the lecture very short, very focused on what the student is about to perform, keeps it relevant, which is key. Telling them what they will be doing, and why, sets the stage. If they make some 'engineering notes', those can be the basis for a short follow up sharing session regarding what worked, what didn't, and how to make adjustments to get consistent outcomes.
Note: I used to employ long lectures....no more. Now, I keep it moving from one learning mode to the next constantly, but all centered on the outcome.
