In the GitHub blog, Josh Black and Marie Lucca explain how the Primer design system moved from CSS-in-JS to CSS Modules. The sx prop’s runtime cost grew with every component on a page, so the team shipped both versions behind feature flags and compared visual regression snapshots. The result: 55% less time to server-side render a page and 25% less time for components to initialize.
github.blog/engineering/architecture-optimization/improving-site-performance-by-shipping-more-css