Cool. I see you're doing a regular (not compensated) horizontal sum in a loop. Horizontal sums are slow, but I'm guessing you wanted to have exactly the same result as if the sum was calculated sequentially (for doubles)? Do you know if any databases use more accurate summation methods (compensated summation)?
There are several accurate summation algorithms; Kahan is one of them. Looks like Postgresql uses an accurate summation method[0] (and not Kahan), so that's probably a big factor in why theirs is so much slower and also makes for an unfair comparison. FWIW I posted AVX2 and AVX512 Kahan summation implementations here[1].
