So my main question, also to myself, is "How much speed are you willing to sacrifice for a new feature?".

Really. Lets assume you have a neat module that deals with your data, and it deals with it pretty well and reliable, but extending it with new features - some of them asked for by others - is getting harder and harder.

We now have git, and making a branch is easy, so you can implements the most requested new feature, or the one that most appeals to you and when you are done and all old tests and new tests have passed, you notice a speed drop.

What considerations do you make to decide whether to release the module with the new neat new feature and mention the slowdown (specified) or do you revert the change and note in the docs that the new feature would cause to big a slowdown.