Making an unrequested change at revision stage to a journal paper I had submitted a paper in a journal, and had got a request for major revision. The changes are mostly related to the experiments, and some language issues. However, while trying to fix those, I stumbled a much better (theoretical) solution to the same problem. Should I include this improved solution in the revised version? Or is it better to write this improved solution as an additional note, and let this paper go as is?
Make the improvement
The review process is there to improve the paper. If there are things that you can do to improve the paper without the reviewers help that's great! Making the changes is helpful to everyone.
From the reviewer's point of view - there will always be comments from other reviewers which I didn't think of. I wouldn't expect all improvements to come from my comments.
From the editor's point of view - they way to publish high quality papers. The review process is one (very important) aspect of this.
From your point of view - you want your papers to be as good as possible.