Flow seems to do some sort of deferred JS loading, and it is slow :/
When you go to a page with flow on it for example https://www.mediawiki.org/wiki/Extension_talk:TimedMediaHandler it takes a few seconds to load the site.
This does not only happen on the main flow page but also pages such as history https://www.mediawiki.org/w/index.php?title=Extension_talk:TimedMediaHandler&action=history
While this loading is happening a user can not interact with the page.
In the case of the history page as an example this JS loading doesn't even seem to change the page content, so why, just why, is it there and why does it take so long?
2.47s main html loaded
5.50s flow content is usable
3.81s main html loaded
5.64s flow content is usable