Are these sites slow because the software on the server-side is slow or because they're JavaScript bloated garbage?

That's a serious question: I find it's often hard to tell what the bottleneck might be in these applications.

The vast majority of slow websites I've seen written with RoR were slow because the DB later want optimised. 1+n query problems, pulling way more data than needed and then processing it in Ruby, missing indices etc.

Either that or by slow views. URL generation is often a noticable culprit.

Most websites are written to minimize developer time not processing time.

They say this and write slow websites because they don't care. They only care about their code being "beautiful" in some weird sense.