What I've seen is that the productive developers are the ones who understand what problem they are solving. They either take the time to think it through or just have an seemingly uncanny ability to see right to the heart of the problem. No false starts, no playing with different implementations. They write the code, it's efficient, and it works.

The slow developers have false starts, have to rework their code as they discover edge cases they didn't think about before, or get all the way through and realize they've solved the wrong problem altogether, or it's too slow at production scale.