(...of course, that doesn't really tell you much about React; seems like all you really learned is that badly architected apps perform badly. Not sure that one needed much testing. And of course, your initial comment seemed to be making a very different claim.)
In any case, our current view is that there is a useful middle ground where React may work well for us. The app in question predates just about all of the modern JS UI libraries/frameworks, at least in anything close to their current forms, so much of the rendering and event management code right at the front end is built with jQuery, early toolkits, and in many cases a lot of home-grown code. All of this still works pretty well, but quite a large proportion of it is functionality that every UI library in the universe provides today, so there's little reason to maintain another version any more. The rest of the app does have a good architecture that has stood the test of time, but for the next big update we're trying to solve much the same issues as everyone else with managing UI code as the app grows.
React is of interest to us for several reasons, from being a useful templating mechanism, through providing a way of composing components that are relatively clean and modular in design, to its efficient DOM updates and declarative nature. These too are presumably the features that make it attractive to many of its users. But it is also of interest for this project precisely because its scope is tightly confined to just the rendering parts of the system and it doesn't try to lock the whole app into its own framework.
We just confirmed early that some of the hype really is only that, and that scaling up to the kind of UI that has more than a few hundred data points on screen at one time is not as easy as some of the advocacy might suggest, so we're exploring how we can use the tool to take advantage of what it clearly does offer and play to its strengths without paying too high a price. In that respect, it's really no different to any other library we'd evaluate.