The way SPAs work is very performant but are also poor for indexing. Critical Path CSS is mostly used in user facing sites which one wants to get indexed by Google. As page speed is one of the factors considered in SEO, and rightly, one tries to optimize that. Vue, React, etc. are poor at SEO as they do not even try to do solve the problem as they are used to build apps and not sites.
I see you haven't used these libraries in at least a few years, because they all have frameworks that work with SEO by server side rendering content. React has NextJS while Vue has NuxtJS.
What speed differentials are you seeing? Since they are server side I don't see any speed differences at all since they just transmit raw HTML and CSS, same as if you would write it by hand.
