Sorry to say, it's nothing React-specific. I understand that there is a React Developer Tools addon for Chrome that might be of interest [0].
As for me, I just turned on Javascript CPU profiling in Firefox Developer Tools, scrolled a bit, and checked what JS calls were causing the slowdown. Because the unminified source for the website is available on Github, you can quickly find and examine the functions' code directly [1].
[0]: https://chrome.google.com/webstore/detail/react-developer-to...
[1]: https://github.com/casesandberg/react-context/blob/gh-pages/...