1. A direct comparison to Facebook's react Github page may not be fair. Their page is 10 times heavier than the blog page - of course its going to be faster.
2. Actually the Facebook's React page is not that well designed when it comes to performance. I see a lot of scripts included in the FB react page blocking rendering (codemirror.js, etc) even in pages where they are not used. If those are removed, the time to first render should drop down by a lot.
3. I feel a more apt comparison would have be comparing to a purely static page like a blog hosted on Github. Not a page which actually includes a live code editor!
4. No one says the page loads in the DOMContentLoaded time. It is mostly a useless metric when it comes to performance measurement.