Seriously, unless you are just an absolute freakin' Game Master Master with 20 years of experience, you just aren't going to get away with 90 minutes of prep each week. And even then, you are going to essentially be relying on rehash. It's not even a reasonable goal. As a DM you should be aiming at least a 1:1 ratio of prep time to game time or you are cheating your players (and probably yourself). If you can't hack that, you can't DM, and I'm still awaiting the exception that disproves that after nearly 30 years of gaming.
I willl add that by far the most often complaint I hear from players from other tables was, "Most DMs don't want to put the work in." I've heard stories about DM pet NPC's, being forced to ride the rails, DM's trying to run the PC's for the players, and DM's with bizarre in game fetishes, and a whole host of other DM flaws. I've never once heard a player complain, "My previous DMs prepared too much (or too well)."
Given the nature of this thread it seems there's an implication here that DMs who don't prep as much as others aren't working hard (and seem to also be destroying the RPG hobby and possibly causing global warmingThe single biggest reason our hobby is dying (if it is dying, and I'm not convinced) is not enough DMs willing to put in the work. There are tons of players out there who would love to play, but can't find anyone willing to run the game, tell the story, build the world, create the adventures, etc.
I think my motto is, be prepared, but also be prepared to chuck stuff out the window as well.![]()
Seriously, unless you are just an absolute freakin' Game Master Master with 20 years of experience, you just aren't going to get away with 90 minutes of prep each week.
The single biggest reason our hobby is dying (if it is dying, and I'm not convinced) is not enough DMs willing to put in the work.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.