An SPA has all the tools in their tooling chain necessary to minimize delivered CSS, yet they're often still slower than a static page web application with a 5000 line CSS file. The issue isn't the CSS rendering, it's in the timing of the resources and their delivery. If you load CSS in 12 different files, especially if you're doing it as the user interacts with the site, then the browser has to combine, reprocess and repaint every line of CSS it has been given so far, each time it gets new CSS. That's after waiting for the network request.

I know I'm talking against industry opinion, but I don't believe browsers are the right platform to be loading things on the fly as the user needs them. It's 50-300+ms absolute best case, plus processing time. It's going to feel bad.