> A world where some websites still don't work properly across web browsers.

The situation today is vastly better than in the days of IE6.

> A world where clean code destroys performance

Clean code does not mean code that is not performant.

In my mind, "clean code" is foremost small methods that do one thing, and are well-described. These small methods will often be optimised away, in the unlikely situation where they were the bottleneck in the first place.

In my experience messy code "for performance" reasons was usually code that had claims to being "fast", but was often incorrect and hard to fix.