The optimization process and recommendations:

1. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
2. Avoid Subselect When Selecting MAX/MIN Per Group (query line: 12): Constant subquery results are usually not cached by the database, especially in non-recent database versions. Therefore, a constant subquery in a WHERE clause will be fully evaluated for every row the WHERE clause will examine, which can significantly impact query performance. Use the method mentioned in the example instead.
3. 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.
4. Replace Join With Exists To Avoid Redundant Grouping (modified query below): When a joined table isn’t used anywhere other than in the WHERE clause, it's equivalent to an EXISTS subquery, which often performs better. In cases where the DISTINCT or GROUP BY clause contains only columns from the Primary key, they can be removed to further improve performance, as after this transformation, they are redundant.

Optimal indexes for this query:

ALTER TABLE `import_data` ADD INDEX `import_data_idx_vendor_importr_sku_profile` (`vendor`,`importRun`,`sku`,`profile_id`);
ALTER TABLE `import_profiles` ADD INDEX `import_profiles_idx_creditornr_id` (`creditornr`,`id`);

The optimized query:

SELECT
        DISTINCT t1.* 
    FROM
        import_data AS import_data1 
    LEFT JOIN
        import_data AS import_data2 
            ON (
                import_data2.sku = import_data1.sku 
                AND import_data2.profile_id = import_data1.profile_id
            ) 
            AND (
                import_data1.importRun < import_data2.importRun
            ) 
    WHERE
        (
            (
                1 = 1 
                AND import_data1.vendor = 'rackmountit' 
                AND 1 = 1
            ) 
            AND (
                EXISTS (
                    SELECT
                        1 
                    FROM
                        import_profiles imp 
                    WHERE
                        (
                            t1.profile_id = imp.id
                        ) 
                        AND (
                            imp.creditornr = 73329
                        )
                )
            )
        ) 
        AND (
            import_data2.importRun IS NULL
        )