Unlike Reddit, logged in and logged out users largely see the same thing. I wouldn't imagine there is much logic involved in serving personalized pages, when they don't care who you are.
The username is embedded in the page, so you can't do full caching unfortunately. But the whole content could be easily cached and concatenated to header/footer.
You also can't do that every time because of hidden, flagged, showdead, etc.
That could be split as a setting in the user-specific header with the visibility part handled client-side. A bit more work, but it's not impossible if it's worth it.
90 to 99% of those are logged-out users, so fully cacheable.
Only a handful of dynamic requests each second remain.