People say "older games" but what they really mean is "the 1 percent of games in mame that run on 3D hardware or have hard drives in them". Those type of games are a tiny minority and just about everything else will run full speed with a 90s era processor (and the majority of those one percent require specialty analog controls that you won't have anyway).
Sorry, but this is a massive exaggeration.
A '90s era processor' might get you some static background classics in an ancient version where MAME was designed with an optimization point of only updating the parts of the screen that changed (an optimization rendered useless and actually slower with uglier code once you got to anything with a scrolling background.
I think you have tunnel vision on this because you personally only care about a tiny handful of games that fall into that category, and are even willing to look past how utterly horrendous things like the Gyruss sound emulation, or colours on many games (due to lack of actual PROM dumps & resistor weight use) etc. was back then. Even emulation of popular systems like CPS1 was full of hacks and glitches for a long time (MT was packed with them) because we didn't emulate the behavior graphic addressing PALs etc. properly and hacked around them.
It's not "the 1 percent of games in mame that run on 3D hardware or have hard drives in them" where you need a good processor. If you want to guarantee a rock solid 100% there are a huge number of titles where you need something decent. While they've not been officially re-included yet the newer Cave titles for example really do need an i5 if you want to avoid framerate drops and those are neither 3D hardware, nor have a hard drive, nor require fancy controls, and are definitely something people are interested in. Plenty of older, but still popular drivers also have high requirements (the properly mixed Batsugun emulation is quite demanding, Sega's System 32 is too, some of the popular PGM titles) This stuff isn't going to run on a 90s processor, it wasn't even emulated in the 90s.
I know people talk about Blitz and some fancy racers, but there's quite a lot more and I'm sure when we fix the remaining (and in places still quite serious) bugs in something like Taito F3 the system requirements will shoot right up too.