Scenario 13: A @PostFilter-protected endpoint listing 'my transactions' has become extremely slow as the transactions table grew into the millions.

@PostFilter runs AFTER the annotated method already fetched the FULL result set, filtering it down in memory afterward — at scale this means fetching millions of rows just to discard most of them. Fix by pushing the filtering condition (e.g., WHERE owner_id = :currentUserId) into the actual repository query, so the database only returns rows the user is allowed to see in the first place.