[rich_harris]

Great question! It's a very common misconception that an app that uses virtual DOM diffing somehow manages to be faster than an app that manipulates the DOM directly.

Re-rendering your entire app (i.e. creating all the nodes from scratch with innerHTML or document.createElement) is too slow for a lot of apps, and is a bad idea for lots of other reasons (losing state in form elements, etc). Virtual DOM just gives you a way to write your app as though you were re-rendering it from scratch, while still being fast enough that you can get away with it.

Svelte is significantly faster than React according to https://github.com/krausest/js-framework-benchmark, because it's taking a more direct approach to the same goal — updating existing elements or only creating/destroying them as necessary. It's also faster than Ember, Angular, Vue, etc. It's not yet quite as fast as Inferno, but it uses less memory.