This isn't quite true. It's only true if you're browing from a computer that is moderately fast (at short timescales before throttling at minimum) and using a b

This isn't quite true. It's only true if you're browing from a computer that is moderately fast (at short timescales before throttling at minimum) and using a browser that does not respect your software freedoms. There are literally hundreds of millions of us out there where executing javascript code on sites made entirely out of javascript can be significant. Weather it's because you use many, many tabs, or browse using an old smart phone. But of course that's not the real cost. That's the computation cost. The real cost of websites switching to an application model directly targeting the DOM is the loss of accessible webpages and the chance that every time you're going to get owned. Weather it's some information leak from speculative execution or something else. Running JS on every page these days is just as stupid as opening every attachment you get emailed to you.