I tried to do something similar in openarena once, but that's far less accurate.

r_slowness is a cvar on the Github-hosted OA engine that gathers all the stats for the rendering similar to r_speeds but then does a bit of math to accumulate CPU slowdown and GPU slowdown with simulated bottlenecks done by sleeping (oh no). I tuned it enough to represent my P2 300 rig and it's just a "mapper's hard realization under spoiled computer circumstances debug mode" cvar and I mainly did it to stress the importance of asset optimization than nostalgic reasons. This was years before PCem became far more capable to meet similar results and having an test-ISO-building profiling routine.

Well, that looks really complicated. I had a 486 with an isa card and performance slowdown was uniform, when it's cpu limited it will be slower in more complex areas. Besides it really didn't felt like running as slow as with a slower cpu.
With the real values it would be accurate, emulating the feeling of a 486 cpu would be very complex and demos and multiplayer should still work.
It's based on Crispy's code and it's a really simple approach, there 35, 60 and 70 Hz are supported. I know in low detail full view there are half of the pixels, i'd just need the real values for the table.
Now only compiles with the Code::Blocks project, should i fix the VS 2008 project? I use VS 2017.
I need to update the demo format but i'm waiting for the PR for 0.99 and other versions support. The source is here:
Edit: link removed.

I've done it myself. The first machine i tried didn't boot and i was welcome with some loud beeps, not the best thing with ear damage.
Most machines don't work now, i've used a k6-2 450 and i had to remove the sound card else i got no display with the trident.
BTW colors were very vivid and contrast was high even on that crt. Nothing to do with modern cheap lcds.

1// Trident 9000i-3
2static const float rates[2][9] =
3{
4    {54.3, 42.9, 34.0, 27.2, 22.0, 18.0, 15.0, 12.1, 11.6}, // high
5    {66.3, 57.2, 48.7, 41.3, 35.0, 29.7, 25.5, 21.3, 22.4}  // low
6};