When you start out on a small app the slowness doesn't stick out anywhere. App keeps growing until one day it becomes very obvious in perf profiles. At that stage it's difficult to refactor out of it to another system, as a lot of the brownfield might've drifted into relying on techniques that can only be used when inserting CSS at runtime: - generate `@media` query selectors with ranges based on JS state and nesting. - calc background color that pass a11y contrast ratio tests based on dynamic foreground colors (for example the color palette generated from an image asset)

While these have new solutions: - @container queries, no need to fiddle with global viewport size calc. - https://developer.mozilla.org/en-US/docs/Web/CSS/color_value...

Even so, it takes time to refactor thousands of files.

>It always was. You decided to make it slow and were fine with it, not sure why that's suddenly unacceptable.

The decision might've been made years before you joined the organization, and you're now left with that decision and have to plan for what to do about it.