I was curious about this so I poked around both and I think I disagree. Both load very fast for me and are snappy and look pretty nice. The one difference is that the Runescape wiki has a single ad in the sidebar or at the bottom, below the content footer. While the Fandom wikis have 3+ ads, far larger, one of which covers content until interacted with (like being closed). For me, Fandom's ad approach absolutely falls within "offensively bad," while the Runescape ad approach reminds me of early 2000s, "here's an ad to pay the bills. We've tried to keep it well out of your way."
So I'd opine that it has less to do with the quality of mediawiki, and more about how much money both Wiki hosts are seeking to gain from the existence of these resources.
Fandom makes it extremely difficult (nigh impossible) to do something as simple as access the page of an image asset.
But being on an old version makes navigating the wiki hard. I'm never sure if some content applies to me. Sometimes they say which version a feature was introduced in, but if a mechanic changes, they often just document the latest behavior.
Interestingly, DCSS's best source of info is the IRC bots/learndb.
Minecraft has this somewhat also, with some people sticking on various versions because of mods, or play style, or combat, etc.
For example, one huge change was going from a 2D map to a 3D one, another was how world generation was done.
See "Eras" here for the big ones: https://dwarffortresswiki.org/index.php/Release_information
Another example is the various Dungeon and Dragons wikis that allow you to toggle between versions, since it has existed for 50 years now.
Games with "always on" or auto-updaters avoid this.
I agree we should be focused on the users, but I think the solution there is not to leave them on various outdated software, but to make it so easy for them to be on the new thing that they are happy with frequent updates.
And I feel pretty strongly about this because I've met people whose entire lives are about keeping old, broken stuff limping along so that pathological bureaucracies can never get their acts together. Sure, it's a living, but it's also a colossal waste of human potential.
I think the theory is people edit more if pages load lightning fast. I can attest to that, especially if you use tools for partially-automated mass edits like https://github.com/wikimedia-gadgets/JWB