I think when designing a language, and a set of libraries for it, the designer has an idea of how code for said language should be written, what 'idiomatic' code looks like.

In that context, the designer can reason about how should code written that way should perform.

So I think this is a meaningful question for a langauge designer, which makes it a meaningful question for the users as well, when phrased like this:

'How does idiomatic code (as imagined by the language creators) perform in language X vs Y?'