First of all, let's please be more considerate of one another. Telling someone they're "full of it" is not helpful to the conversation.
two said:
You can whip up a generic CR 25 NPC with the absolute minimum required stats in 15 minutes, maybe. But this isn't a full (or even a partial) stat block by any means, and worse yet:
The CR25 NPC you come up with will be totally cookie-cutter.
The NPC will be incredibly weak; unoptimized; slap-dash; and not much fun (I suspect) to pound into the ground.
What about magic items? You simply can't slap those together for a CR25 NPC. You must think about it; minimally, how did the fighter-type make it so high vs. high level magic? Does he specialize in anti-magic? Are his saves through the roof? Does he have other tricks?
What about movement? Ranged attacked? Flying? Ethereal perhaps? Skills? Don't get me started on feats -- how many feats does that CR25 fighter type have anyway? What about classes? Ranger2/Rogue2/Fighter21? Something else?
What about tactics? Ways to take on (for example) a run-of-the-mill 18th level wizard?
You simply can't put all this together in 15 min. Impossible. You just can't. At least, if you don't want all your NPC's to look, feel, act, and do the same things.
It seems you're assuming that all NPCs should be statted up like PCs if you want them to be effectual. I disagree. They are effective for their purpose - which is to challenge the PCs without killing them outright. I won't go to the attacks and damage, since that doesn't concern you as much as special abilities. Even with special abilities, you can make opponents with greatly different feel. A Spiked-chain wielding Ogre warrior will feel very different from a War Troll, especially when the PCs keep getting tripped from 20 feet away with one, and have to come up with some acid attacks to have a HOPE of killing the latter.
A high-level wizard only needs about 6 spells per individual NPC - if he's an opponent, he likely won't last more than 10 rounds anyway. And if one wizard casts a series of lightning spells, while another focuses on holding spells, the players nor you don't need to know what his cantrips are, nor his 1st or 2nd level spells, for that matter - they only need to know about the evocations he cast, or if you have a player who's a counterspeller, the spell he's currently casting.
I do not forego stat blocks completely, as I said before - when I stat up a major NPC, I usually use a program such as PCgen, or a piece of paper, and stat him up fully, because I put as much into him as the PCs do theirs. But it's doable either way.
Whatever I, S'mon, or other DM's here on this thread say will likely not convince you, but all I can say is that I can do it, I've seen it done, and I just played a game this weekend where half of the opponents were DM'ed in just this fashion (a round-robin DM game with maximum 5 minutes prep time).