DAX simple measure issue

Try something like this:

Order_20 = 
  COUNTX(
    DISTINCT(
      SELECTCOLUMNS(
        FILTER('Fact Order'[Items]<=20),
        "__Order_Key",[Order_Key]
      )
    ),
    [__Order_Key]
  )

The "Filter" statement on a fact table that large will lead to performance issues. The good news is that you can use a simple filter in this CALCULATE formula instead. Try this:

Order_20 = CALCULATE ( DISTINCTCOUNT ( 'Fact Order'[Order_Key] ), 'Fact Order'[Items] <= 20 )

Thanks!

Matt Drabik