Here is an interesting story about the flarum forum dev and their switch from ember to mithril due to extensibility problems and performance. Hopefully the auhor will blog some more about his experiences. Mind the bloat if you do not want to sink the boat.
You didn't provide a link. I assume you were going to link the blog post announcing their switch to Mithril — that didn't contain any rationale behind it.
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()))
]);
}