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.

No, I remember the web from the modem days, and I'd be quite happy with pages from the modem era but loaded over 100Mbit Ethernet.

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.

It's not just processing power, it's all the round-trips and timeouts and social buttons delays.

And advertising.

Dumb banners or text adverts would be pretty fast, but all that script-loaded, dynamically retargetted, real time bidded crap is pretty slow.

Except dumb banner and adverts are NOT pretty fast and a frequent cause of the spinner still running long after the page loads.

Meaning they are not dumb enough.

Without which we wouldn't need all the processing power and 640K of memory really would be enough.