It obviously doesn't transfer 20MB of anything. With a hot cache it needs about 300KB of traffic to load the initial thread list.

Browsers caches that kind of assets. So in practice you only only download it once every time it gets updated

Unless you only use web applications in private browser windows and/or clean your browser data every time you close the browser and/or have cache disabled (though this one's more by accident, since Firefox gives you the checkbox under network tab, but it doesn't work for just the site that you're developing locally, but seems to persist globally for all of the browser).

Of course, that's not the use case of most people, but that's also one that shouldn't be dismissed entirely - for people who don't trust the modern web in any capacity, local system software is indeed a better choice.

Which is likely more than once a day given today's web-development practices (continuous integration and such).