How can i write a measure witout Summaize for performance

Question

Hello,

I have a measure that use a summarize like as bellow :

CALCULATE (  
    [Cnt],  
     SUMMARIZE (  
        Sales,  
        DimProduct[Product_id]  
    )  
    ,SUMMARIZE (  
        Forecast,  
        DimProduct[Product_id]  
    )  
)  

The definition of the measure Cnt as bellow :

Cnt :=  DISTINCTCOUNT ( Invoice[CustoID] )  

I want to rewrite the first measure without using Summarize to have a best performance .

Any one have an idea how can i do it ?

Thanks for help !