Depends on the requirements. Writing the code in a natural and readable way should be number one.

Then you benchmark and find out what actually takes time; and then optimize from there.

At least thats my approach when working with mostly functional languages. No need obsess over the performance of something thats ran only a dozen times per second.

I do hate over engineered abstractions though. But not for performance reasons.

Almost exclusively. Was doing Scala for 5+ years, followed by F# now for the past 5 years. But im also doing React and sometimes node.js etc when needed.

I really love FP, although i tend to write more pragmatic code, and follow the principle of “functional core, imperative shell” rather than using imo overly complicated free monads and the likes.