EF Core is not slow at all, especially if you disable change tracking, which you don't want in many cases anyway.
That was long time ago and EF improved a lot since then.
But it can be slow if used improperly. But hand written SQL can be slow too if not carefully crafted.
Sometimes small changes in fragmentation, statistics or data composition will trigger a different queryplan which will affect your query immensely.
Good monitoring and a DBA nearby would be your best bets to overcome this. The point is that using a relational database is not a trivial thing once the database becomes large. Until then its relatively simple.