Improve very slow measure

[ucciod]

I have this measure

Delta Movimento =
VAR DefaultDate = DATE(1900, 1, 1)
VAR ConfDate = SELECTEDVALUE(DimPurchaseOrder[ConfirmedShippingDate], DefaultDate)
VAR MovDate = SELECTEDVALUE(FactWmsInventory[MovementDate])
VAR ReqDate = SELECTEDVALUE(DimPurchaseOrder[RequestedShippingDate])

RETURN
SWITCH(
TRUE(),
ConfDate = DefaultDate, DATEDIFF(MovDate, ReqDate, DAY),
DATEDIFF(MovDate, ConfDate, DAY)
)

I use it in a table with 15 field from different tables, I can not modify the data model (as in add calculated columns) so i need to use a measure but this one takes minutes. How can I improve this?

[lbendlin · Accepted solution]

Use DAX Studio to analyze the query plan and server timings.  Verify and adjust your data model and your measure accordingly.

DO NOT use 1/1/1900 as a default date.  Do not use default dates. Leave the value blank.