Enjoyment increases with the amount of prep I do. But it's because I spend my time prepping a toolkit instead of a straitjacket. I think a lot of GMs have a tendency to prep the wrong things -- things that are not only wasting their prep time, but which actually make them
less prepared for the game because they're limiting their options instead of giving them more options.
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.
I dunno.
Consider the One Page Dungeon. I'm skeptical of its general utility as an ideal, but as an exercise to remind yourself that prep can be kept to a minimum as long as you're prepping the right stuff it can be very valuable. And I really don't think you need that much experience in order to get a really solid night of entertainment out of a dungeon with 16 or so keyed locations described in a single page.
And if you can't write a page of text and whip up a functional map in 90 minutes, you're doing it wrong.
I dunno. Maybe I'm over-estimating people. Maybe I'm secretly a genius. But I've been able to say, "Wanna play?" and whip up a fun, functional dungeon in 10 minutes while the players are rolling up their characters. And I've been doing that since I first started playing.
I guess my point is this: In your prep you can write a single bullet point "when Nerissa learns that the PCs are investigating the murder, she'll send a squad of 12 orcs to attack them". That took... what? 12 seconds to prep? But it'll probably chew up at least 15-30 minutes of game time.
The one place I will invest "extraneous" prep time is in the gilded edges: Handouts. Digging through supplements to assemble tomes of unique lore which can be found in the campaign world. Customizing a miniature for a particularly notable bad guy. These chew up a lot of time, but they're nice extras if you can find the time. But I don't think they're essential for a successful session.
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.
See, I'd argue the opposite: The single biggest reason our hobby is dying is the belief that being a GM requires a huge time investment and a devilishly complicated set of skills. And I think that's a belief primarily driven by GMs doing bad prep instead of effective prep.