I've maintained a ~20k LoC re-frame app for a year and a half. The performance of Clojure's persistent datastructures with pervasive sCU gives you generally good performance but it's not panacea. I've debugged a hundreds-of-milliseconds freeze on laptops from a poorly written hierarchical menu and I get pauses of 70-100ms if enough data comes in on our main view.

Using it with cljs-time and storing times in the app state is a really easy way to shoot yourself in the foot perf-wise since that's based on closure's date object and two equal date objects are not identical. This fails the fast identity check but will pass the structural equality check so no vdom gets generated but the check is not that much cheaper than a diff.

This is also a B2B app where we have no demand for mobile use so I haven't had to do load time or mobile perf optimization. If I did, the first thing I'd be concerned about is the bundle size since the cljs runtime plus React represents a fairly sizeable amount of overhead.

Not to say that reagent+re-frame is bad, it's just not so amazing you don't have to care about perf. I think Reagent would run particularly well on top of Inferno since the library provides lifecycle events to function components and I did experiments back in April and June on Inferno with persistent datastructures to good results. I just don't want to maintain it.