Best low-level optimization tutorials?

I had found a resource around 4 or so months ago either on here or reddit that was amazing. It took some matrix problem, solved it naively, and then applied different optimizations such as branchless coding, simd, openmp, compiler optimizations, etc. It showed the cumulative effect of each optimization, and I though it was amazing. Unfortunately, I lost the link to this website and cannot find it despite hours of Googling and searching on HN and Reddit directly.

Wondering if anybody here knows resources that were useful for similar or different topics, or might be able to provide a link to what I lost.