The optimization process and recommendations:

1. Avoid Subqueries (query line: 4): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
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'.
4. Replace In Subquery With Correlated Exists (modified query below): In many cases, an EXISTS subquery with a correlated condition will perform better than a non correlated IN subquery.

Optimal indexes for this query:

ALTER TABLE `table` ADD INDEX `table_idx_id_product_id_datelast` (`id`,`product_id`,`datelast`);

The optimized query:

SELECT
        AVG(time) 
    FROM
        (SELECT
            UNIX_TIMESTAMP(max(table.datelast)) - UNIX_TIMESTAMP(min(table.datestart)) AS time 
        FROM
            table 
        WHERE
            EXISTS (
                SELECT
                    DISTINCT 1 
                FROM
                    table AS table1 
                WHERE
                    (
                        table1.product_id = 12394 
                        AND table1.datelast > '2011-04-13 00:26:59'
                    ) 
                    AND (
                        table.id = table1.id
                    )
            ) 
        GROUP BY
            table.id 
        ORDER BY
            NULL) AS T