I just read a post on Academic Jungle on how much time it takes the blogger prof. (tenured female) to prep a 1-hour lecture. She is in a STEM discipline (science, tech, engineering, math). Wow, those STEM folks live in a very different world. Entry-level undergrad that she's taught before--30 minutes prep. Yes. That's what she wrote. Same for entry-level grad course--30 minutes (or up to 1 hr for upper level grad)!
In my world, we don't use textbooks. We use books and articles, even for lower-division courses. And I have to skim (or even re-read) those readings before class. Then I go over my powerpoints and notes. I am a much more effective teacher if I put in this time (average of 1.5 - 2 hrs per hour of class for a class I've taught before). I brush up on my facts plus think about how to explain and expand upon concepts and theories. Plus, mine is a rapidly moving field and I need to significantly update my lectures every time I teach.
So let's see: 1.5 hrs x 3 hrs/wk x 2 classes x 15 wks = 135 hrs per semester!!! 90 to 135 more hours per semester for my course prep compared to what I am assuming (perhaps incorrectly) is fairly typical in the STEM disciplines. Interesting. And of course, I'm comparing in terms of a 2-course per semester load, so comparing across research universities (if those in the STEM world do teach 2 classes--my neighbor in engineering teaches one course/semester).
135 hours--that's a lot of hours. Teaching prep clearly varies significantly across disciplines within my university. But no one ever talks about it.