That's because the jit compiler in the Julia standard library selects the blas for you. As a programmer you don't have to optimize beyond using idiomatic type strategies. You could, say write a galois field type and use the \ solve operator to solve a Reed Solomon Erasure using old fashioned lu decomposition (automatically without pivoting, even), and then turn around and use the same \ operator with floats one line down and it will give you highly optimized blas.