Is it just JMAP, or why does Fastmail's web app feel so fast? I have moved away from all locally running mail apps to Fastmail and even fetch/alias all my other mail accounts to them because of the much better experience.

Because most working web developers actually have no idea how to write JS; they follow what is presented (perpetuated) as industry standard practice, but in a React-and-NPM world, "industry standard practice" means bad practices.

Previously:

<https://news.ycombinator.com/item?id=24218967>

<https://news.ycombinator.com/item?id=33794755>

Probably partially because of JMAP, partially because they have a single mission and know when to leave well enough alone