I haven't tried this software, but I imagine it's more about having the UI keep up, so you can stay in the flow state.

Is raw code output so large that your editor can't "keep up" really something to optimize for? For me personally I write as little code as possible. you don't get bonus points for number of lines you write.

Yes, while at the same time having a responsive UI is important for editing, and we're not hanging less stuff in our editors than yesterday. Both emacs and neovim and vscode are essentially IDEs today.

Yes it is, especially when you work with lots of large files in big codebases. I can open files in Vim that would crash VSCode, which is essentially why I still use Vim for certain parts of development.

Presumably speed is an overall important factor for the creators — not just startup speed — and I'd imagine it translates into speed elsewhere, such as input latency and UI responsiveness.