Discourse has run into a pretty nasty performance issue on Android lately [0]. It would be too strong of a critique to say they are already having the problem you mentioned, but surely if they shipped this as a finished problem and had this issue it would be a big deal. It doesn't appear to be a fundamental problem with their codebase, as the video comparisons to iOS show that it runs reasonably smoothly there. This shows that JavaScript performance on mobile has a ways to go yet.

[0] http://meta.discourse.org/t/why-is-discourse-so-slow-on-andr...