This morning, with AdBlocker switched OFF, I am experiencing slow loading.
Turning Firefox’s Web Developer tool ON shows instantly that it headlines “Waiting for advintage.net” repeatedly. (i.e excessive delay waiting to download an advert.)
Using the developer tool to dig deeper into how long each element of a page takes finds examples like this, note “unknown duration” and status “waiting for response“, all advintage.net, everything else reasonably quick:

After turning AdBlocker back ON my performance returns to normal. It’s because AdBlocker tells the browser to ignore links to adverts found inside a webpage, so my view of the forum isn’t delayed by advintage, and the forum flies.
The evidence against advintage.net is strong. A problem with advintage.net, either server or network.
Of course, SOD’s Law being what it is, possible this is a double fault. Something out-of-spec done by the forum that upsets advintage.net. Although the calls look ok to me, the code is complicated!
Dave