20% is a low enough bar that you can cite a well-known example: Objective C vs. Swift removes entire sections like headers and adds sorely needed typechecks. Ou

20% is a low enough bar that you can cite a well-known example: Objective C vs. Swift removes entire sections like headers and adds sorely needed typechecks. Our internal results are less code and fewer defects per line of code. However, a large number of people haven't switched. I don't blame them either; the transition (split code base) is painful and Apple makes things worse with language churn. For your 5x-10x case, what is true is that the gain is genuinely possible, but it is just as likely to come from libraries than it is language constructs. Because of that it's often 5x-10x in a limited area.