A much more efficient version of this takes about 1/6 of the processing:

SELECT G.OrderDate, G.ProductKey, QtyOrdered = SUM(F.QtyOrdered), G.MTD

FROM

(

SELECT

ProductKey

, OrderDate

, MTD= (SELECT sum(QtyOrdered) FROM #t WHERE OrderDate <= a.OrderDate AND ProductKey = a.ProductKey)

FROM #t A

GROUP BY ProductKey, OrderDate

) G

JOIN #t F

ON G.ProductKey = F.ProductKey AND G.OrderDate = F.OrderDate

GROUP BY G.ProductKey,G.OrderDate, G.MTD

ORDER BY G.ProductKey,G.OrderDate;