Ok, I did a few more test on my 486 system with different CPUs I've retrieved from my stash... Same Zida 4DSP board, same Verite V2100 @ 55Mhz core clock. For software mode I switched to a ET6000 VGA since the Verite VGA/VESA mode is a bit too slow and buggy.
AMD 486DX4 120MHz (WB cache version)
Quake 1.08 Demo1 (UniBE 7.1, VESA 320x200, Fastest FPS in software mode) : 13.7 FPS
VQuake 1.07b5 Demo 1 (320x200) : 16.9 FPS + 3.2 FPS
Intel Pentium Overdrive S 83 MHz
Quake 1.08 Demo1 (Same as above 320x200) : 21.8 FPS
VQuake 1.07b5 Demo1 (320x200) : 24.6 FPS + 2.8 FPS
Cyrix 5x86 120MHz
Quake 1.08 Demo1 (Same as above 320x200) : 14.4 FPS
VQuake 1.07b5 Demo1 (320x200) : 18.2 FPS + 3.8 FPS
Cyrix 5x86 120 MHZ + Optimizations (BTB=on, FPFAST=on, LSSER=off)
Quake 1.08 Demo1 (Same as above 320x200) : 16.5 FPS
VQuake 1.07b5 Demo1 (320x200) : 20.5 FPS + 4 FPS
With the exception of the Pentium Overdrive, none of the other 486 class CPUs, even when paired with a 2nd generation Verite doing some of the rendering operations, are enough to play Quake at a decent framerate.
I've also recorded the framerate at 512x384 and 640x480, but there's a noticeable framerate drop, so it's hardly worth reporting. VQuake is still too dependant on the CPU and it's not using the second generation Verite chip to its fullest. The situation may change using GLQuake but that would require a Windows installation (and with only 32MB of RAM available, there would be other bottlenecks under Windows).
The Cyrix 5x86 120MHz with optimizations on is much better than any 486, including the AMD 5x86. I'd really love to try the "mythical" Cyrix 5x86 133MHz, but I've yet to see one. For this test I used a brand new (still in its original tray) Cyrix branded 5x86-100GP manufactured by IBM which is also stable at 120Mhz. I also wanted to test the Pentium Overdrive at 100 MHz, but my sample is not stable enough at that frequency; Quake 1.08 and VQuake crash on startup; oddly enough Quake 1.06 manages to complete the timedemo once or twice before crashing or hanging. Last, my AMD 5x86 (ADZ stepping) progressively became more and more unstable at 160MHz despite a 'generous' cooling (I'm using a Socket 5 heatsink and fan), so no more testing on that chip at 160MHz. I guess it's time to hunt for a replacement...