In my experience, the great majority of perceptible slowness comes from badly-written JavaScript and running too much code, generally not even code that touches

In my experience, the great majority of perceptible slowness comes from badly-written JavaScript and running too much code, generally not even code that touches the DOM, though certainly touching the DOM too much in bad ways (and structuring the DOM in bad ways) often contributes to it. Certainly the web as a platform is slower than a precisely-built thing honed for speed for a particular use case, but speaking generally the web is only perceptibly slow when you write bad code (or are waiting for the network). Which is admittedly rather common.