Fanaelialae
Legend
There were a confluence of issues specific to 3e that made these things particularly problematic for that edition.@Fanaelialae: Yeah, but none of the issues you describe is particularly new to 3e. If you were running a 1e AD&D module and there was a wizard in it, they'd probably have a list of his prepared spells, but if the issue of a spell book came up, it would still be up to you to decide if that was all the spells in the spellbook or not. And if you weren't running a module, you'd need to work with the spells which is the single biggest part of prepping a spellcaster. Spell lists are not an issue particular to 3e chargen.
And neither is picking out relevant gear to ensure minimal effectiveness in an NPC "boss monster".
And while 5e has somewhat simplified this, everything you talk about is still relevant particularly if you are playing a game with feats and magic items and thus complicating your balance issues. And to the extent that you can take short cuts, those short cut techniques have been around since 1e days. Probably the absolutely best short cut in both cases is simply having a stock character of appropriate level to serve as a template for any character minor enough you don't want to stat them out.
I mean think about this, in AD&D it wasn't that unusual to be using a random encounter table and for the random encounter table to have the result "NPC party". The expectation there is that you can just wing and entire other party of adventurers after a minute or two of dice rolling.
In 2e, there was no expectation of magical gear by and large. You could make an effective high level fighter to challenge the party in a few minutes. He didn't need magic weapons or armor to be effective. Your biggest decision point was picking out a weapon for him to be specialized in. Very different from 3e.
Casters were also far easier to prep because they got significantly less spells in 2e. In 3e it was not uncommon for a high level caster to be pre-buffed with a half dozen or more spells if they had time before combat. In 2e, it was rare to see more than 2 or 3 spells precast, IME.
As for the party issue, I always just assumed that DMs wrote up the NPC party during prep, although I never actually ran any of those modules and therefore never had to deal with it myself.