>I do have a UPPER(LastName) index in my table.
>For some reason rushmore doesn't work in this case and I still havn't found the reason why.
>
>I tried it with a view it's doing the exact same thing...
>The strangest thing is that it only take to long in certain searches and not all.

In order for Rushmore to work the left side of you filter expression must exactly match an index expression. In you case

* The index expression 
UPPER(LastName) 
* The filter expression should be 
SET FILTER TO UPPER(LastName) = < ... >