While parsing time may not be an issue on client-side, on server-side the situation is a bit different, "server side is done in milliseconds" does not mean it is faster than client side, cf. this presentation [1]. Moreover, I thought that Flux was the solution to the latency introduced by the multiple HTTP requests.
When this thread started off with the comment saying apps being isomorphic/universal leads to them being "SEO friendly and fast rendering times on first visits" it wasn't about React rendering speed, it was about page load times. First-time visits mean when there the browser hasn't cached various page elements.
[1] http://www.slideshare.net/nickdreckshage/react-meetup