This is probably true for anonymous (not logged in) users, but might not be if you are logged in. Things like user/community/instance blocks, votes, read/unread status, etc. need to be accounted for when generating the main feed for logged in users, and those are dynamic. It might be cached for some time, but the retention time would likely be short (a couple minutes maybe).

I haven’t worked too much with the caching that piefed does, but I know it keeps some info in a redis cache to help speed things up (blocks, votes, etc.). That way getting all that info doesn’t need to be a new db query every time except maybe the first time. Some pages also make use of etags so that if the content is unchanged, the request doesn’t need to hit the db at all. I imagine lemmy has some mechanisms to do caching as well, but I am unfamiliar with the code at that level.