As far as subsequent page loads are concerned, I would be very surprised if there are any major differences in speed, since Discourse, NodeBB and Flarum are all single page applications built on the same principle, which is that once the web app bundle is loaded it’s all speedy low-bandwidth data from there, since there’s very little redrawing of pages going on.
As @Jay_Pfaffman pointed out, probably the only noticeable difference is the first-time load. Our bundle is bigger than that of NodeBB or Flarum, which impacts the loading time of your first-time visit to a Discourse site. The reason our bundle is bigger is that Discourse simply does more advanced stuff out-of-the-box. For instance, I’m pretty sure we’re one of very few community platforms that track reading time, which is a very valuable metric for our pseudo-autonomous moderation system.
Speed is a major priority for Discourse and we consider it critical to our UX story, but our overarching goal is to facilitate civilized discussion in the best way possible. If some of the fancier things we do to achieve that goal means we won’t be the fastest at everything, we’re okay with just being fast enough.