> In general database-backed sites struggle to implement if-modified-since since a fully correct implementation has to traverse the graph of all database objects that are looked up in the request which costs about as much as… the request.

That's only true if your implementation favors editing performance over GET request performance ... in which case you get what you ordered. If your feed requires any database connections then that's a choice you made.

I disagree that HTTP caching is complex. The basic primitive are:

- An indication until when the returned document can be used without checking for updates.

- A way to check if a cached document is still valid (based on either modification time or id).

This provides everything you need to implement an efficient feed reader.