I think it’s a server side cache thing. I was loading a specific old topic on that instance and it took very long to load, but once loaded in the first time refreshes were decently fast (even in private windows). I’ve observed the same behaviour on the Figma forums, which are also discourse.

Measure the initial load time, subsequent loads are indeed faster

And to test many replies you actually have to see them since they're not loaded on page load, only when you get to the bottom of that page, so just opening the top doesn't test anything re. lots of replies