"Very very slow" might be a bit of an overstatement. The initial React render for that page takes 200ms on a MacBook. A release build will speed that up slightly, and cut down the bundle a little, but in the scheme of things it's not going to be noticeable to the end user.
If you actually wanted the page to feel fast you would probably do something like <script>document.write(window.navigator.userAgent)</script>
I did not expect to argue in HN of all places whether it's OK or not to run the development mode of a library in production when it's so simple not to and the authors are trying so hard to prevent people from doing that.
LOL, that's prettymuch what the previous version of the site did. The new one is fancy partially to learn some new things, and partially to enable the host/share feature.
If you actually wanted the page to feel fast you would probably do something like <script>document.write(window.navigator.userAgent)</script>