Any SQL query can be slow, especially if you have a large database and/or use a lot of Foreign Keys within your database.

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.