Performance

The query didn't run when you wrote it. It ran when you looked — and it ran again when you looked again. Deferred execution, multiple enumeration, and the boundary that quietly drags a whole table into memory. Most of these traps reproduce with a List<T> and no database at all.

It returned in 40ms on your machine. In production it takes nine seconds and fires 2,001 queries. Nobody wrote 2,001 queries — EF Core did, because your local database has five rows in it and has been lying to you since the day you cloned the repo.