Optimize queries

I have the following function

CREATE FUNCTION [dbo].[SuiviRupture]
    (@CodeArticle [NVARCHAR](13),
     @CodeSite [NVARCHAR](5), 
     @CodeStructure [NVARCHAR](13))
RETURNS @calcul TABLE (CAMOY FLOAT, QTEMOY FLOAT)
AS 
BEGIN
    WITH temp AS 
    (
        SELECT 
            t1.[datecol], t1.[Prix de vente TTC],
            t1.Quantité 
        FROM 
            [V_VentePromo] t1
        INNER JOIN 
            (SELECT DISTINCT 
                 [datecol], [Code Article], [Code Structure],
                 [Code Site], 
                 ROW_NUMBER() OVER(PARTITION BY [Code Article], [Code Structure], [Code Site] 
                                   ORDER BY [datecol] DESC) AS rn
             FROM 
                 (SELECT DISTINCT  
                      [datecol], [Code Article], [Code Structure], [Code Site]
                  FROM 
                      [V_VentePromo] t2
                  WHERE 
                      [Code Article] = @CodeArticle 
                      AND [Code Site] = @CodeSite 
                      AND [Code Structure] = @CodeStructure) g
          ) a ON a.datecol = t1.datecol
                 AND t1.[Code Article] = a.[Code Article]
                 AND t1.[Code Structure] = a.[Code Structure]
                 AND t1.[Code Site] = a.[Code Site]
        WHERE 
            t1.[Code Article] = @CodeArticle 
            AND t1.[Code Site] = @CodeSite 
            AND t1.[Code Structure] = @CodeStructure
            AND rn <= 28
    )
    INSERT @calcul 
        SELECT
            CASE WHEN COUNT(distinct [datecol]) = 0 
                    THEN 0 
                    ELSE SUM(convert(float, Quantité)) / count(distinct [datecol]) 
            END as QTEMOY,
            CASE WHEN COUNT(distinct [datecol]) = 0 
                    THEN 0 
                    ELSE SUM(convert(float, [Prix de vente TTC])) / count(distinct [datecol]) 
            END AS CAMOY
        FROM 
            temp
     RETURN;
END;

I call this function by this query

SELECT  
    t1.[Code Article], t1.[Code Site], t1.[Code Structure], 
    u.QTEMOY, u.CAMOY
FROM 
    V_distinctVente t1
CROSS APPLY 
    dbo.[SuiviRupture](t1.[Code Article], t1.[Code Site], t1.[Code Structure]) u

The execution time is too long, I have a number of rows of V_distinctVente about 10 000 000. How to optimize it?How could Icombine main query with the T-SQL code of the TVF in one query?it's executed row by row in the main query