I did find this however, which goes into details about why Ember was not a good choice for them: http://discuss.flarum.org/139-introducing-flarum-s-fast-new-...

Seems to be a well based choice. Doesn't seem to have anything to do with performance though.

While Mithril is probably a lot more extensible and easeier to work with since there's a smaller API surface, seeing "templates" like this makes me sad:

  view() {
    return m('div.text-editor', {config: this.element}, [
      m('textarea.form-control.flexible-height', {
        config: this.configTextarea.bind(this),
        oninput: m.withAttr('value', this.oninput.bind(this)),
        placeholder: this.props.placeholder || '',
        disabled: !!this.props.disabled,
        value: this.value()
      }),
      m('ul.text-editor-controls', listItems(this.controlItems().  toArray()))  
    ]);
  }