The parent commenter earlier seems to be implying that it's only a matter of not caring.

> care so little about the performance of the code they ship to browsers.

> but I'm curious to hear how do you know it for backend code but not frontend code.

Because I find backend languages extremely easy to reason about for performance. It seems to me that when I write in a language like rust I can largely "grep for allocations". I find that hard to see in javascript etc. This is doubly the case because frontend code seems to be extremely framework heavy and abstract, so it makes it very hard to reason about performance just by reading the code.