Non Distinct Count Calculation Efficiency

Nice, Owen!! Great variations here!

My notes: 

First of all, notes about the period table: 

    CALCULATE (
        < Some Measure >,
        ALL ( 'Period Table' ),
        'Period Table'[Period Index] >= startPeriod,
        'Period Table'[Period Index] <=curPeriod
    )

Typically, instead of the above, which I have used in the past, I have been using

    CALCULATE (
        < Some Measure >,
        DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
    )

 This seems to be slightly faster. I also have been using it when I might have some other filter active that I don't want to strip away, and only want to strip away the date (so avoids using ALL). I tested the two variations above on your version #5, and performance was slightly improved (6.3 seconds vs 7.2 seconds) - that was giving the wrong data, but the efficiency test was still accurate. I'll explain more below

I also want to be clear that I'm also using this for the "all customer" calculation here: 

VAR allCust = 
CALCULATE(
    DISTINCTCOUNT('All Order Table'[Customer]),
    DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
)

When running this on its own, it takes only about 1.5 seconds. So I know there isn't a problem with this part of the code. 


So speaking of the versions, Version 5 was definitely the best yet at 6.3 seconds!
Just to be clear, here is what the code looks like with the modification

VAR RepeatCustomers =
    GENERATE (
        CALCULATETABLE(
            VALUES ( 'All Order Table'[Customer] ),
            DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
        ),
        VAR FirstOrder =
            FIRSTNONBLANK ( 'All Order Table'[OrderNbr], 0 )
        VAR LastOrder =
            LASTNONBLANK ( 'All Order Table'[OrderNbr], 0 )
        RETURN
            EXCEPT ( FirstOrder, LastOrder )
    )
VAR NumRepeat =
    COUNTROWS ( RepeatCustomers )

This took only 6.2 seconds. But then I realized the data was incorrect. Again, because the FIRSTNONBLANK was removing the filter context of DATESBETWEEN. So I had to add another calculate there. This threw us back down to 26 seconds calc time, and it looked like this: 

VAR RepeatCustomers =
    GENERATE (
        CALCULATETABLE(
            VALUES ( 'All Order Table'[Customer] ),
            DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
        ),
        VAR FirstOrder =
        CALCULATETABLE(
            FIRSTNONBLANK ( 'All Order Table'[OrderNbr], 0 ),
            DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
        )
        VAR LastOrder =
        CALCULATETABLE(
            LASTNONBLANK ( 'All Order Table'[OrderNbr], 0 ),
            DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
        )
        RETURN
            EXCEPT ( FirstOrder, LastOrder )
    )
VAR NumRepeat =
    COUNTROWS ( RepeatCustomers )

This version blew my mind a little bit, Owen! I didn't realize you could write VAR INSIDE a calculated virtual table?!?! It really threw me off. Can you help me understand a little bit how this works? 
I think the "GENERATE" calc throws me off a little bit, because I haven't used it much before. Only generate series. 
I see that you first create a one column unique table of customers.  Then my hunch here is that the FirstOrder "VAR" table gets applied at the row level to the unique customers? Or is it creating another table next to the first and applying a filter from the first of the customer? This is throwing me a bit. And then the EXCEPT is working on FirstOrder and LastOrder table, but then it applies the filter back to the customer table? 

Version 6 and Version 7 - these I understand how you're calculating, but again, really smart usage here. Version 6 took about 19 seconds, but 6 was only at 14 seconds. I would not have thought of using calculate (didn't think it was possible) as a filter argument! Genius! Unfortunately, again, I had to add the DATESBETWEEN context to get this to work. And I had to add it to the VALUES table and to the CALCULATE formula. So to be clear, here is the final version:

VAR RepeatCustomers =
    FILTER (
        CALCULATETABLE(
            VALUES ( 'All Order Table'[Customer] ),
            DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod)
        ),
        NOT CALCULATE ( COUNTROWS ( 'All Order Table' ),DATESBETWEEN('Period Table'[Day],startOfPeriod,endOfPeriod) ) = 1
    )
VAR NumRepeat =
    COUNTROWS ( RepeatCustomers )

As for building out a PBIX, happy to do so, but I can't post the files on the forum - it won't let me.. Thought you could. Anyway, I can send you a PM and could email it to you, or if there is another way to upload here, happy to do that as well.