A year (and some Discourse upgrades) later, and the situation does seem to have improved a bit.
It’s a bit anecdotal, but it does seem to be more usable on an older phone.
Same here, crawling on desktop browser and it feels like getting slowe as the threads build up over time not to speak of long threads, thankfully there’s the near invisible to me (white background) short scrollbar to drag the thread down to the last post. Opposed, snappy on mobile phone, model circa 2021 on Android.
Bit of testing, dark theme on Mozilla Firefox with their Gecko engine is substantially faster than Vivaldi which uses the Chrome engine. Short thread scrollbar is much better visible.
I’ll let you in on a secret: one of OSM Americana’s developers (yours truly) just finally moved on from a more-than-decade-old Mac on its last legs. Me clutching to that computer hasn’t done much to keep the application from getting more bogged down and bloated than we’d like. I’d contend that feedback from mobile phone users serves as a better forcing function anyhow.