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Very little typescript changes your code in a way that would be meaningful for performance. I think enums are one case. It's not like it changes your loops or adds object copies or anything like that

Typescript is pretty much a 1:1 mapping to "modern" Javascript plus type annotations. It will only do non-trivial code transforms when compiling to an old JS version.

Of course one could argue that Javascript itself doesn't allow much control over performance.

Do you know any specific cases in which typescript generates badly performing code as part of transpiring to JS? Any cases that aren't due to targeting older JD versions for browser compatibility?

Ridiculous take. TypeScript turns into JS practically one-to-one. In 5 years I’ve never heard of a performance problem introduced in transpilation.