MDX Using Aggregate really slow

Hi!

I've got a MDX Query which is performing quite good (100ms) but as soon as I use an aggregate function it slows down dramatically (64000ms).

I tried the following:

WITH MEMBER [Sales Order Number].[Branch].[Others] AS Aggregate([Sales Order Number].[Branch].[Branch])
SELECT NON EMPTY { 
[Measures].[Amount In EUR], 
[Measures].[Quantity] } 
ON COLUMNS, 
NON EMPTY { (
CASE @SalesAdministrator
WHEN '[Sales Administrator].[Sales Administrator].[All]' Then [Top Sales Admins]
ELSE STRTOSET(@SalesAdministrator, CONSTRAINED)
END,
CASE @Customer
WHEN '[Customer].[Short Name].[Other Customer]' THEN [Other Customer]
ELSE STRTOSET(@Customer, CONSTRAINED)
END,
[Sales Order Number].[Branch].[Others]
 ) }
DIMENSION PROPERTIES MEMBER_CAPTION, MEMBER_UNIQUE_NAME 
ON ROWS FROM ( SELECT ( 
STRTOSET(@MonthNumber, CONSTRAINED),
STRTOSET(@ProductGroup, CONSTRAINED),
STRTOSET(@FinancialCustomerGroup, CONSTRAINED),
STRTOSET(@SalesAdministrator, CONSTRAINED),
STRTOSET(@Year, CONSTRAINED)
)
ON COLUMNS FROM [Sales Order])

Is there something wrong? 

The main target is to filter the result with the calculated member (of course the aggregate looks different then).

Thanks!