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From a technical side, it probably means that Reddit is showing static pages to visitors while HN is showing dynamic ones.

Reddit's previously stated that non-logged-in users cause substantially lower resource consumption per user - I imagine that's a more likely explanation.

Reddit uses an edge-cache for all logged-out content. They pass logged-in content.

A cached HTTP document in something like Varnish should take virtually no resources whatsoever.

HN is on Cloudflare but they might not be caching pages at the edge (they should).

Presumably there's only so much caching you can do it this case - the points change on articles every second. Not like everyone has to see the most up to date content but it would be a trade off (that many wouldn't like)