A RefactoringBoundary.

If you make a change to improve the performance of a program, is this a refactoring?

I see optimization and refactoring as two separate things, even though they often use the same transformations, and a particular transformation you do to your program may be both.

The reason I see them as different is because they have a different purpose. You do refactoring to make the code easier to understand, you do optimization to make it go faster. Introducing a variable (for example) could be done for either purpose, but depending on how you do it you are primarily doing one or the other. When you are refactoring you are thinking about making the code clearer. Your judgement on whether it's successful is based on your (subjective) assessment of whether the change makes the program easier to understand. When you are optimizing you are thinking about performance. You should be using a profiler, before and after the change to ensure that your optimization really did help performance. If the situation is performance critical you should keep a log of your change so you can retest its effectiveness when your environment (compiler, VM etc) changes later on.

So although the two are similar, and share many transformations, I see them as different because their purpose is different.