Optimizing UDF - Is Using DUAL Expensive

Question

I am new to Oracle technology (primarily versed in SQL Server) and I am having trouble with the performance of a UDF.

The purpose of the UDF is to take some inputs for an "event", and look up an "expected time for event b to occur". The function returns a single DATE value.

The first thing that occurs is a SELECT ... INTO [variable] FROM DUAL; a variable. Finally a lookup occurs in a reference table (only ~30k rows), and that is stored in the return variable.

It took 8 hours to process on 800k rows. I can select all of the columns without using the function in less than 40 seconds.

My main question is: Is the use of DUAL costing me?

We are on an Oracle 11g RAC.