If it turned the inner loop into a closed form expression, I would expect the outer loop to go through 10k iterations a lot faster than needing half a second.

Running the C code through Godbolt with the same Clang and -O3 optimizations it looks like you're right, there are still 2 loops being performed. The loops are unrolled to perform 4 iterations at a time but it's otherwise the same. Other than that it didn't do too much with the loops. Hilariously there is a god awful set of shifts and magic numbers... to save a single modulo operation from running once on the line declaring 'r'.

See here for why the Go version performs worse https://news.ycombinator.com/item?id=42251571