There is a small piece in one of my books about this. I'll do some reading and get back to you tomorrow likely. The SCD transform is process intensive because it has to hit the table for each row and cannot cache that table. So it is for sure not fast, but I think there is an alternate design to avoid the SCD transform, for these situations.

In the meantime I would also mention indexing the PK and associated businessKey in both tables if not already done (clustered is best). But I'm sure you have that covered.