I used to share this sentiment (and I’m a web performance consultant by profession so very few people care about performance as much as me!), but when you consider how much calculation we _happily_ let our JS do at runtime, I don’t think forcing CSS to be static/preprocessed is worth it. And that’s not even me taking a swipe at overly-JSsed front-end; I’m talking about any runtime work that JS picks up.
Is preprocessed CSS faster? Yes. Is it meaningfully faster? Probably not.
Is preprocessed CSS faster? Yes. Is it meaningfully faster? Probably not.