This upgrade was really a way through hardships. It failed at the first try, and it wasn’t easy at the second try as well. It wasn’t easy for Belarusian (Taraškievica) Wikipedia, too.
First, there was a problem with bits.wikimedia.org rewrite engine which couldn’t process addresses with dashes include (be-x-old is so be-x-old, btw. when will it be moved to be-tarask?).
Second, ResourceLoader is a good thing indeed. Moving accessory stuff to the end of the page is a good idea, but the page rendering process is not so fine for my eye now. For a moment I can see the page with no styles applied and JS’s executed.
For now, Belarusian (Taraškievica) Wikipedia feels better and serves as the most substantial and outstanding source of knowledge in Belarusian.