Why is CSS-in-JS slow?

CSS-in-JS solutions like Styled Components and Emotion CSS allow for complex stylesheets using JavaScript knowledge, but they come with performance downsides. Traditional CSS parsing in the browser is quick but not free, and CSS-in-JS adds additional steps that can slow down rendering. Some modern tools like StyleX and PandaCSS mitigate these issues by compiling CSS at build time instead of runtime. Understanding these nuances is crucial for selecting the right styling approach for your project.