This question hasn’t received any attention, so maybe it’s a real oddball.
Since posting, I’ve experienced the same behavior—stalled page load—right here on Meta, which is no doubt running an optimal setup.
This happened when browsing on both my desktop and my phone. When it occurred on the phone, I switched it from wifi to LTE and the page instantly finished loading.
All this has me thinking it’s an issue with my own internet config/connection. Though it’s puzzling that it only happens with Discourse sites.
I’d still be interested to hear if anyone has seen something similar, or has general troubleshooting advice.