The MHz ratio really understates the difference.

66MHz probably wasn't an 80486DX; as far as I can recall, those only went up to 50MHz. It would have to be a 486DX2, with the RAM running at only 33MHz. https://netlib.org/performance/html/dhrystone.data.col0.html says that a 486DX2/66 was about 31 to 51 Dhrystone MIPS.

https://www.riscvschool.com/2023/03/09/t-head-xuantie-c906-r... says the T-Head (Honey Badger) C906 RV64 core is about 2.3 Dhrystone MIPS ("DMIPS") per MHz, so we should expect the 480MHz C906 to be about 1100 DMIPS.

So the clock speed is only ≈8× higher, but the actual speed gap is probably closer to 32×.

And, for vectorizable workloads, we should expect the V extension to give us another 4–8× boost, so 128×–256× the DX2/66, which didn't even have MMX.