I'd say that in-memory/not-in-memory isn't the big difference - it's whether your database is in-process or not. Even with just a database running on the same node, but in a different process, connected to via unix socket, the context switches alone lead to very different performance characteristics. Actually going over the network obviously changes more. It's very easy to miss antipatterns like N+1 queries when you test on sqlite but run on a shared database in prod.