This was first time reported on the Finnish Wikipedia on November 2017 when Firefox 58 was still a beta version (discussion). There is no way to reproduce, as the slowness happens randomly.

It's not related to any user rights, and where should be the favicon, there is the "loading" icon showing. And even if you have many tabs open (like fi.wikipedia.org, wikidata.org, meta.wikimedia.org), when one of them is not finished loading, none will finish. And if you do a ctrl+shift+r on one of those sites, then all of them will finish loading at the same time. Also, if you open the "Inspect elements" window, all of those sites get loaded and you will never encounter this problem while "inspect elements" is open.

Everything was working well on Firefox 57, so I'm sure it's related just to Firefox 58.