A lecture is a performance. When at the front of the class, I always felt like I was on stage, and accordingly, I was always "on". I was conscious of everything

A lecture is a performance. When at the front of the class, I always felt like I was on stage, and accordingly, I was always "on". I was conscious of everything I said, and every movement I made, because everything went into the performance. That is one of the main reasons that giving a lecture is tiring - it's work to be "on" for over an hour. The other main mental work is, as you said, devising strategies for explaining material to students who don't understand it the first time. It requires patience and self-control (if you betray any sense of frustration, you will lose them) on top of the difficult task of figuring out what their exact misunderstanding is. And I agree, it's harder to do the more basic the material gets. Explaining advanced systems concepts to computer science juniors was much, much easier than explaining basic programming concepts to people with no prior programming experience. The more knowledge and concepts you have in common with your students, the easier it is to appeal to things you know they already understand - and you have the advantage of being able to use some of your areas jargon.