The optimization process and recommendations:

1. Avoid Correlated Subqueries (query line: 11): A correlated subquery is a subquery that contains a reference (column: DT) to a table that also appears in the outer query. Usually correlated queries can be rewritten with a join clause, which is the best practice. The database optimizer handles joins much better than correlated subqueries. Therefore, rephrasing the query with a join will allow the optimizer to use the most efficient execution plan for the query.
2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
3. Explicitly ORDER BY After GROUP BY (modified query below): By default, the database sorts all 'GROUP BY col1, col2, ...' queries as if you specified 'ORDER BY col1, col2, ...' in the query as well. If a query includes a GROUP BY clause but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying 'ORDER BY NULL'.

Optimal indexes for this query:

ALTER TABLE `BACS_KongPrima` ADD INDEX `bacs_kongprima_idx_dt` (`DT`);
ALTER TABLE `BACS_MinuteFlow_1` ADD INDEX `bacs_1_idx_dt` (`DT`);

The optimized query:

SELECT
        dbo.BACS_KongPrima.DT,
        AVG(dbo.BACS_KongPrima.DP_H2O) AS Tx,
        (SELECT
            AVG(dbo.BACS_MinuteFlow_1.Abs_P) / 1000000 AS expr1 
        FROM
            dbo.BACS_MinuteFlow_1 
        WHERE
            (
                dbo.BACS_MinuteFlow_1.DT = (
                    SELECT
                        MAX(dbo.BACS_MinuteFlow_1.DT) AS Expr1 
                    FROM
                        dbo.BACS_MinuteFlow_1 
                    WHERE
                        dbo.BACS_MinuteFlow_1.DT <= dbo.BACS_KongPrima.DT
                )
            ) 
        GROUP BY
            dbo.BACS_MinuteFlow_1.DT 
        ORDER BY
            NULL) AS Px 
        FROM
            dbo.BACS_KongPrima 
        GROUP BY
            dbo.BACS_KongPrima.DT 
        ORDER BY
            NULL