Virtual Key Figures question

When coding ABAP for virtual key figures, a case ... select statement is done on the datatarget --> to choose the right one.

It's though time consuming as it's called for every query you run against the cube!

Does anybody know if there is a way to limit the user exit for a specific query? In my opinion, it's acceptable to have a query slower, not all queries for the cube ...

Thank you for any help or advise

Ioan