See T8575 for a brief mention. Feed/notifications have a tangled mess of a data fetch plan right now:

• We load all the stories.
• We load all the primary objects + transactions.
  • I believe ObjectQuery sometimes misses transactions (incorrectly discards them in a way that seems "at random", likely causing T8525).
  • Loading the transactions reloads many objects:
    • Transactions load their primary object, first.
    • I believe ObjectQuery frequently misses the workspace cache.
    • Transactions then load all their handles.
      • This should be off by default.
      • This should use handle pools.
      • I think this misses the workspace cache, too.
• Now, we load all the required handles.
  • We already did a lot of this work.
  • This misses all the caches.
  • The earlier handle loading should happen here instead.
  • All of this should use handle pools.

In some cases, this ends up looking like: "Load transaction > load object, return; load handle > load object, return; load object; return; Load handle > load object, return". And I caught 6 loads on one profile instead of "only" 5, so that's not even the entire story.

Some of this is relatively easy to disentangle, some of it is a bit messier. Particularly, making transaction query default to not loading handles may be a bit of a mess.