Doom uses fixed point math so it doesn't matter if you have an FPU. But the fastest 486SX was only 33MHz, which isn't really fast enough for Doom, and they were typically used in cheaper systems with slower graphics cards/buses, which also made a big difference.
One of the biggest contributor to performance differences was the implementation of the L2 cache on the motherboard.
More expensive chipsets tended to have better cache implementations... Assuming the system even had cache installed, those cheaper systems often paired a 486SX with a cheap motherboard and zero L2 cache.
When installed on the same motherboard, same cache/memory config, with the same graphics card, same bus speed, a 486SX should run Doom identically to a 486DX.
I remember it being the case that Doom was aggressively tight-loop optimized, so the small amount of L1 cache was more significant especially on clock-multiplied 486s. Though you are right that the SX/DX distinction wasn't meaningful for Doom as it was all done with internet mathematics.
Yeah, the clock multiplier will also make a large difference.
Which might actually be a fair comparison, The DX2 and SX were launched at the same time, so there is a decent chance the "fast 486 DX" someone is talking about is actually a DX2.
The SX2 was launched later, with the same 2x clock multiplier and same size L1 cache, so would preform identically to the DX2 (in non-fpu tasks like doom). But the DX4 was launched at the same time, now with a 3x multiplier and L1 cache doubled to 16KB....
