Anti-joins can be expensive. Index join columns. NOT EXISTS often faster than LEFT JOIN + IS NULL. Test both approaches.

Index join columns on both tables

Compare LEFT JOIN + IS NULL vs NOT EXISTS

Use NOT EXISTS for large tables often faster

Filter left table with WHERE before join

Check execution plans

Checking foreign key instead of primary key for NULL

Using INNER JOIN instead of LEFT JOIN

Not indexing join columns

Confusing anti-join with semi-join

Not testing performance with real data