> And in the browsers, you rarely do heavy processing. If you are, you're doing it wrong - that logic needs to live on the server.

This is very incorrect. At my last company we had a React interface (a specialized IDE, really) that needed to juggle (sort, filter, process) and work with sometimes hundreds of thousands of entities at once, all in-memory. JavaScript did this just fine, and the user experience (and the API design) really benefitted from not having a bunch of extra round-trips to the server.

Even in this extreme use-case, the bottleneck was always the few-hundred items that were actually rendered in the DOM at a time. We virtually never had performance problems from sheer volume of underlying data.