Mostly due to developers writing code that synchronously waits for network requests to complete before updating the DOM. The user clicks, then nothing happens w

Mostly due to developers writing code that synchronously waits for network requests to complete before updating the DOM. The user clicks, then nothing happens while the app sends a request, and app only updates the DOM after the request completes. It's that 'nothing happens' step that makes things feel slow. They do other things like rerendering part of the app every time any part of the state changes rather than limiting the updates to just the bit that matters to that component too, so the app is doing a ton of unnecessary work (React helps here; it ignores state updates that don't change anything). There's a lot of problems in web apps that make them feel slow, but they mostly distill down to developers following some bad practises that are easily avoidable. Web apps that are slow because they're maxing out what the framework is capable of are very, very rare. Making a faster framework won't fix the ones that are just coded badly. Making a more intelligent framework might.