Browsing could never be slower than what we had without modern CSS/JS: desktop applications. If you think we've lost anything (speed, convenience, whatever) with CSS/JS, then you don't remember (or willfully forgot) what the web was like in the 90s.
But that was because we were all using dial up, now the problem is the processing power required to crunch the overly-abstracted CSS/js output of our huge and complex toolchains.