Bad performance in calculating datediff without weekends/holidays

The rules are these:

1. You should avoid as much as possible calculated columns in DAX, especially in fact tables. 

2. When you have to put in a calc column, you should refrain as much as passible from using CALCULATE.

3. Do the calculation in the source or in Power Query.

There are too many reasons behind those statements to be able to expand on them here. If you want to know their source, please read The Definitive Guide to DAX by The Italians.

If you really want to do what you should not, you can try this:

DateDiffWithoutWeekends =
// First of all, you should calculate
// things once only, never repeat
// the same calculation
VAR MinDate = MIN( Facttable[Date1] )
var MinTime = MIN( Facttable[Time1] )
VAR MinDateTime1 = MinDate + MinTime
VAR MinDateTime2 = MIN( Facttable[DateTime2] )
RETURN
	COUNTROWS(
		FILTER(
			HulpDatumTijd,
			true()
			&& NOT( HulpDatumTijd[is Weekend] )
			&& HulpDatumTijd[Datum] >= MinDate
			&& HulpDatumTijd[Datum] <= MinDateTime2
			&& HulpDatumTijd[DatumTijd] > MinDateTime1
			&& HulpDatumTijd[DatumTijd] < MinDateTime2
		)
	)