the idea of cost-free came from C++ where the belief is that you don't pay for what you don't use. The underlying meaning there is that you don't pay the cost

the idea of cost-free came from C++ where the belief is that you don't pay for what you don't use. The underlying meaning there is that you don't pay the cost of virtualized function calls if you're not using polymorphism (this is a big reason why C++ doesn't feature virtualized function calls by default). polymorphism is of course an abstraction, the idea of cost-free is that you only pay for the abstractions you actually use. This is not the same as maximizing performance. The real question is whether or not you can do these things without giving the developer the control to maximize performance, and I think the answer is going to be no. The response will be that it's "good enough", but that's not actionable as we don't know what "good enough" means for any particular project.