Out of curiosity how did you find that out? I've been looking for a decent debugging workflow for React.
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/...