Wait what? It is insanely complex to make a performant web app compared to a performant C app (let's say we're using a traditional UI toolkit as a comparison).

Wait what? It is insanely complex to make a performant web app compared to a performant C app (let's say we're using a traditional UI toolkit as a comparison). You need to understand the DOM (and how various CSS declations contribute to rendering complexities), the rendering model, reflows, work around them, pre-fetch tricks, etc etc. Not saying anything like this impossible, but I'm pretty sure the claim that "oh all web devs can do this" is invalid. Basically; the performance issue with declarative UIs is that it's really easy to declare something very complex by mistake.