> be slower than pure js There's no way of knowing, without knowing what they were doing previously. The whole point of React is to leverage the virtual DOM to

> be slower than pure js There's no way of knowing, without knowing what they were doing previously. The whole point of React is to leverage the virtual DOM to avoid unnecessary (and very slow) DOM manipulations. Given the sheer number of things Slack has on any given page it isn't unreasonable to expect that React will make things quicker, even if it has a penalty at startup time.