Memory issues with DAX filter

Hi,

I have a model where I have a Customers table and an Orders Table. I want to add a new mesure in the Customers table where i want to compute the average buying cyle. So i add this DAX mesure:

   BUYING_CYCLE = 
            AVERAGEX(ORDERS; 
                    DATEDIFF( CALCULATE ( 
                        MAX ( 'ORDERS'[ORDER_DATE] ) ;
                            FILTER(ORDERS;  EARLIER('ORDERS'[ORDER_DATE]) >= 'ORDERS'[ORDER_DATE] )
                    ); 'ORDERS'[ORDER_DATE]
                    ; DAY )
            )  

This works fine but it's not correct. I should add a second filter like this:

FILTER(ORDERS; EARLIER('ORDERS'[ORDER_DATE]) >= 'ORDERS'[ORDER_DATE] ) && EARLIER('ORDERS'[ID]) <> 'ORDERS'[ID] )

But if I add this second expression the query is really slow and the memory gets to 98 plus %.  There aren't a lot of records in the Orders table, like 25000. 

Can anybody tell me how can i understand what is happening here?

Thank you.