Days Sales Outstanding (DSO) calculation performance issue?

We have written an abap that dumps out data for a credit rescoring check process. As part of the process we are taking the KNB1 data and capturing the current DSO for each account via the function CUSTOMER_DSO_CALCULATION. The code is being extremely cpu intensive and we found that the problem is that calling this function for each of our 1million plus customers is getting extremely poor performance due to the sql in the function. I did check OSS and there appears to be nothing out there for this item.

Does anyone know if there is a different way to do this calculation?