Learning is by definition a "stressful" experience, in that obtaining new knowledge stresses the brain. However, there's a difference between eustress ("good stress", e.g. doing a Sudoku puzzle) and distress ("bad stress", e.g., being yelled at by your boss). Constant bad stress especially decreases your ability to learn, but is actually beneficial in small doses. See this Nature article for a good overview: https://www.nature.com/articles/npjscilearn201611 There have also been studies that taking mini-breaks from stress when learning help improve retention: https://www.ninds.nih.gov/News-Events/News-and-Press-Releases/Press-Releases/Want-learn-new-skill-Take-some-short-breaks
I try not to make my lectures or explanations too dense, and make random digressions to provide a "mental break". For instance, in my software engineering classes, when I discuss how sometimes the best code is no code, I'll do a quick talk about Arthur Schopenhauer's philosophy of antinatalism ("th