It does seem surprising (and I was all ready to slate rust until I remembered to build for release). If you care that much - naive FORTRAN iteration on the same

It does seem surprising (and I was all ready to slate rust until I remembered to build for release). If you care that much - naive FORTRAN iteration on the same machine, same compiler, took 1200 s. (I am, obviously, taking inspiration from "There's plenty of room at the top": https://news.ycombinator.com/item?id=23442123 https://news.ycombinator.com/item?id=23442123. Naive FORTRAN's slow even compared to C there, but it's not the same machine anymore so not apples to apples. Also, as a small point - I initialised all matrices with random numbers. Below half a second and that might start to be a significant time demand.) I commented on this thread, because it definitely seems to be in the realm where the compiler matters. So there is opportunity.