I managed to shave off another couple of seconds and set a new best SuperPi 1M time again.
K6-3+ @ 4.5x135 Mhz, Gigabyte GA-5AX, 256 MB PC133 CL3 @ 135 CL2.

Still no luck with running SuperPi @ 4.5x140 as the memory module the board likes best isnt really capable of 140 MHz CL2, at least not in my hot apartment. It can wait until the cold season.
I diddnt bother with running Sisoft Sandra at 4.5x135 but that is just as well as the setting isnt 100% stable, the PCI bus runs a bit to far out of spec for the Voodoo III. Here is a screenshot with the memory bandwidth at 4.5x130 which is 100% stable, not too bad.

Here are some rather bad FPS numbers with the PCI Voodoo III, the Gigabyte Ga-5AX really seems to hate the card.
K6-3+ 5x100, 256MB CL2, Voodoo3 "3500" PCI.
640*480 16bit
GLQuake: 58.5 FPS
Quake II standard OpenGL driver 640*480: 37.5 FPS
Quake III: 34.7 FPS
Unreal Gold: 44.7 FPS
K6-3+ 4.5x130, 256MB CL2, Voodoo3 "3500" PCI.
640*480 16bit
GLQuake: 76.3 FPS
GLQuake with miniGL 1.47: 89.3 FPS
Quake II standard OpenGL driver: 48.2 FPS
Quake II 3dnow! standard OpenGL: 53.4 FPS
Quake III: 43.3 FPS
Unreal Gold: 55.5 FPS
K6-3+ 5x125, 256MB CL2, Voodoo3 "3500" PCI.
640*480 16bit
GLQuake: 73.4 FPS
GLQuake with miniGL 1.47: 90.9 FPS
Quake II standard OpenGL driver: 47.8 FPS
Quake II 3dnow! standard OpenGL: 53.2 FPS
Quake III: 43.6 FPS
Unreal Gold: 56.5 FPS
K6-3+ 5x120 256MB CL2 Voodoo3 "3500" PCI, Onboard cache on.
640*480 16bit
GLQuake: 76.2 FPS
GLQuake with miniGL 1.47: 94.2 FPS
Quake II standard OpenGL driver: 49.3 FPS
Quake II 3dnow! standard OpenGL: 56 FPS
Quake III: 44.3 FPS
Unreal Gold: 58.0 FPS
The Quake II numbers with 3Dfx miniGL 1.47 was 2-3 FPS better than the ones with 3dnow! + the standard OpenGL driver, at best 59 FPS.
All FPS numbers posted here are with the games running at 640*480 but the scores at 1024*768 were not much worse.
I really have no idea why this system performs this bad with the PCI Voodoo III. I have found the screenshots I thought I lost with some results from last year when I benched the exact same hardware down to the memory module with a Geforce 2 MX and a Geforce 3 ti200. Those two video cards performed as I would have expected, the PCI Voodoo III did not.
New PC: i9 12900K @5GHz all cores @1.2v. MSI PRO Z690-A. 32GB DDR4 3600 CL14. 3070Ti.
Old PC: Dual Xeon X5690@4.6GHz, EVGA SR-2, 48GB DDR3R@2000MHz, Intel X25-M. GTX 980ti.
Older PC: K6-3+ 400@600MHz, PC-Chips M577, 256MB SDRAM, AWE64, Voodoo Banshee.