Add all that up and you might find it takes a few hours for each session. Not minutes. And as I stated before, if I want to improv or just glance at the characters and play the stubborn dwarf who isn't trying to convey information related to an entire book's worth of material, then no problem. My prep time is zero seconds. But, again, to run it as intended, takes time.
You are right though. If I want to read it, scribble some notes, improv each character, read the text box the AP provides, and force the player's down the railroad. You got it. 15-20 prep time per session, max. And I am also saying, there is nothing wrong with that. I've done it. I imagine every experienced DM has done it. It can be fun. But, for the long haul, as a player and DM, I prefer my DM to have things thought out.