SSAS MDX performance tunning

Dear guys,

How can fine tune the following MDX query. Currently it takes 1mmin25sec to run which is too much...

I am trying to get for each Pos Stratum (kind of point of sale Category) the best sale.. the query works but it's too slow and I am looking how to bypass the crossjoin i currently have...

SELECT 
		GENERATE
		(
			EXCEPT(
					[Point Of Sales].[POS Stratum].[CAT POS STRATUM].MEMBERS,
					{[Point Of Sales].[POS Stratum].[CAT POS STRATUM].&[-],
					 [Point Of Sales].[POS Stratum].[CAT POS STRATUM].&[?],
					 [Point Of Sales].[POS Stratum].[CAT POS STRATUM].&[]
					 }
				)
			,TOPCOUNT({[Point Of Sales].[POS Stratum].CURRENTMEMBER * [Point Of Sales].[POS Name].children},1
					,[Measures].[Sales Amount]
				)
		)
		 ON 1
		,[Measures].[Sales Amount] ON 0
FROM 
	[CUBE LNL DM]
WHERE
	{[Calendar].[Calendar - Monthly].[CAT CALENDAR YEAR].&[2012]}

The bottleneck is the crossjoin between POS Stratum and the Pos names. 

Thanks in advance

Miloud Bel.