@Phranque: I'm checking, but the indexes are on the id and serial - and yes they are both primary key, this is the same on both tables.
@lifeInAsia: Yeah, checked over that too; id_number for both tables is int, and serial is varchar.
There are around 5000 rows on each table, so there is bound to be some delay as each entry is being evaluated, I appreciate that.
My intention is to be able to select from the serial table ONLY serial numbers that aren't in use yet.
Thanks as always for the continued support.