Reviewable founder here. Yep, Reviewable is a heavy client-side app, and I make no apologies for it. Mobile devices are getting powerful very quickly (you can already use Reviewable pretty much fine on modern phones/tablets) and the target audience of developers generally needs reasonably powerful devices anyway. I'd rather miss out on some users in the short term than lock myself into an obsolete server-centric architecture in the long term...

That said, performance is always an issue, it's just a question of where to set the target.