I think switching from Styled-Components over to CSS Modules would be an easy performance win. The CSS is loaded as separate minified files rather than served with the JS allowing the the page to paint quicker.

A mix and match approach would be fine while Strapi is transferred over I think the build routine would need to be updated to accommodate this though. I'm aware CSS Modules can't read from the theme object directly I think we'd need to store all variables as CSS variables too (again this has a performance boost).

I have a helper function package that makes CSS module classnames a little easier to read (in my opinion) but of course you can use CSS modules without any extra packages too.