Nested SUMX performance impact within Tabular cube

Assuming you have a business logic in your measures that require the SUMX approach (I cannot be sure of that), you could improve the performance by avoding nested loops. Replace ForecastTable and ShippingTable with the name of the tables containing data using by the corresponding measures.

Forecast Accuracy :=
IF (
    [Forecast Amt] >= [Shipped Amt],
    0,
    1
        - (
            DIVIDE (
                SUMX (
                    SUMMARIZE (
                        ForecastTable,
                        'Customers'[AccNo],
                        'Calendar'[Year],
                        'Calendar'[Month],
                        Items[Supplier],
                        Items[ItemCode]
                    ),
                    [Forecast Amt]
                ),
                SUMX (
                    SUMMARIZE (
                        ShippingTable,
                        'Customers'[AccNo],
                        'Calendar'[Year],
                        'Calendar'[Month],
                        Items[Supplier],
                        Items[ItemCode]
                    ),
                    [Shipped Amt]
                ),
                0
            )
        )
)