It's easy enough to do a basic implementation, but getting good numerical stability and good performance is hard (and in Javascript, it's pretty much impossible

It's easy enough to do a basic implementation, but getting good numerical stability and good performance is hard (and in Javascript, it's pretty much impossible with current implementations). See also the matrix multiplication benchmarks in the post: JS is 60x slower than Matlab, even though it's already using typed arrays. A naive triply nested for loop in C would probably perform similarly to the JS, which is to say a lot slower than something optimized for the characteristics of the processor (number of registers, vectorized floating point and cache sizes mostly, I'm not sure if Matlab is using multiple cores here).