Cyrix 486, UMC 486SX.

Cyrix should be slightly slower, UMC probably faster. No clue about 1x multiplier though

Yeah, actually looking at it, UMC is faster - https://www.youtube.com/watch?v=kQEdPQDXdLY

So original Cyrix 486 is the slowest option available.

I'm not so sure the Cyrix would be the slowest. It did get the highest score in Speedsys ..

Hmmm.

The tangible, real-life differences between Cyrix and Intel/AMD 486s are negligible at best, not counting Cyrix's 486SLC/DLC parts, or the 486S parts with the smaller cache.
Sometimes the benchmarks paint one in favor of another, but the real-life effect is far too small to be noteworthy.

UMC's U5SX OTOH is a good deal faster per-clock than Cyrix, Intel or AMD 486SX's, but motherboard support tends to be a little spotty, at least according to RedHill.au's guide.

Right. I am really just looking for the lowest Speedsys scores for now. The other benchmarks show a different story so I am just concentrating on this one for now.

And cache being enabled vs disabled could completely change the end result in respect to which processor returns the lowest score.

This motherboard doesn't even pretend to have official support for the Cyrix 5x86. It works, but no 5x86 optimizations are enabled and it thinks it is a 80487sx. I can enable at least some optimizations with one of the register changer programs and it seems to work just fine despite no BIOS support.