How to make it faster?

I have two tables containing varchar fields:

tableA          tableB
A1               B1
A2               B2

I have a query with a correct function but it's too slow:

SELECT B1, B2 FROM tableB
WHERE B1 + B2 IN(

        SELECT DISTINCT subA1 + subA2 FROM tableA
        WHERE A1 + A2 not IN(
                SELECT B1 + B2 FROM tableB
        )

I have to concat two fields, because they are composed primary key. The problem is: This query is very slow (tableA is very big).

When I run the query in Query Analyser i'ts fast, but when i have it in my stored procedure and run the stored procedure from Query Analyser, i'ts slow. (it becomes fast when I exclude B2, subA2 and A2 from the query so i'ts the combination of the two fields that makes it slow)

I've tried different ways to make it faster. By replace the inner IN( function by a JOIN-function, it becomes 10% faster, but not fast enough. You can't replace both IN( functions with JOIN right?

Best regards!

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