I'm not saying you can't, I'm saying that since the code that eventually gets executed the same way just one layer down, it's not useful to JUST consider the performance.

You're not using TypeScript to get faster JS execution. You're using TypeScript to get types in JS. Compare development speed and avoiding bugs together with performance in that case. Same for Clio.

Ok, but I still want to know how much of a performance tax I am paying to know whether it’s the right tool for the job. If it’s a thousand times slower, maybe that overwhelms the benefits of extra features.

And what if every statement I write in the upper language compiles down to one loop that runs a hundred times? The target language is a lower bound, not an upper bound.