Wolfenstein 3D, May 1992 -> Doom, December 1993 -> Quake 1, June 1996 -> Quake 2, December 1997
In the span of five years, they went from:
Wolf: https://i.imgur.com/1gGewDp.png
Doom: https://i.imgur.com/6CqvrLg.png
Q1: https://i.imgur.com/6wZg8NG.png
Q2: https://i.imgur.com/4Lxtg4f.jpg
The leap forward between the release dates of Doom and Quake 2 in just four years is particularly impressive.
I remember some years after quake 2 was out (99ish ?) a friend showing off his new computer by actually playing quake 1 with all the bells and whistles turned on.
So yes the engines did develop quickly. Much quicker than the PCs of the day in fact.
*being school kids, these weren't gaming PCs, these were for 'homework' and 'research'.
When Wolf3D came out in May 1992, its minimum spec targeted the installed base of 286s, and a 50Mhz 486 was bleeding edge. When Doom was released in Dec 1993, you could buy a Pentium. (And if you kept it till '96, that Pentium could manage Quake at 320x200).
I agree there was a bifurcation in the late 90s when 3D hardware acceleration arrived (see the colored lighting and texture filtering in the Q2 screenshot). Quake 1 was getting patched with hardware rendering up until Quake 2's release, likely the bells and whistles your friend showed off :)
Still, Q2 was perfectly playable even on below-average PCs at the time of release, if you were willing to stick to the same resolution as Doom (i.e. 320x200).
Wolf, 1992: 386, 4 MB memory, 8 MB disk
Doom, 1993: 486, 8 MB memory, 40 MB disk, Sound Blaster Pro/compatible
Q1, 1996: 75 MHz+, 32 MB memory, 128 MB disk, SBP/c, GeForce 6200 LE / Radeon Express 1200
Q2, 1997: 90 MHz+, 32 MB memory, 32 MB disk, SBP/c, 4x CD-ROM, GeForce 6200 LE / Rage 128 Ultra 16 MB
Which isn't to knock the awesome folks at id at all! Just, it was a heady time on both the hardware and software sides.
Most of those are on the other side of the pre-internet chasm though, so not much accurate info.
Thanks for correcting!