I cannot speak for c++, but I recently rewrote an optimised c routine in assembly. A 2-4x speedup obtained. I might have gotten partway with careful tuning of

I cannot speak for c++, but I recently rewrote an optimised c routine in assembly. A 2-4x speedup obtained. I might have gotten partway with careful tuning of the source, the way the parent suggests, but I could have not gotten all the way.