Use a fast language and forget some of the old advice from "12 factor apps," some of which are focused on avoiding common challenges when scaling low-performance platforms.

D isn’t “big” (as in “reddit big”), and any web-1.0 forum that I remember from 2000s worked like that on old hardware. If you’re curious how a single SQL database can serve so fast, it’s caches and invalidation when a thread CrUDs itself. Old boring tech.

Edit: the tone was a little sharp, edited it out, my apologies

Its open source iirc it uses MongoDB and redis for caching, worthy of noting is that you can also browse and download that entire board with a Newsgroups client. It also posts to IRC as well.