html is indeed codegenerated. See display-items, newpoll-page, etc.
pg has said in a few places that as load has grown, HN has cached commonly-requested data just enough to keep performance mediocre. Arc helps flexibly-yet-declaratively specify what to cache and when to evict it. Look at instances of defcache, then at its implementation. There's probably other sorts of caching; this code is several years old and we don't know how it's been changed as HN has grown.
"How are requests being handled?"
I don't know enough about what you know to answer usefully. Are you aware of defop?
