I think 25% is a low estimate. Using a proper programming editor alone could realistically offer 2x or more productivity over a basic text editor, and there have definitely been programmers who stayed with basic editors.

And I have first hand seen programming teams where there was clearly more than a 25% difference — some could code much, and some could barely code at all.

I think it would be quite fair to say that, between tools and individual skill, there could easily be a 5x speed difference between slower and faster programmers, maybe more. Granted, LLMs are even faster, but I don’t think a 5x potential speed up was a slouch.

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.