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. Avoid Subqueries In From Clause (modified query below): The database cannot properly optimize subqueries in the FROM clause. Therefore, we recommend to extract the subqueries to temporary tables, index them and join to them in the outer query.
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. 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'.
5. Use Numeric Column Types For Numeric Values (query line: 22): Referencing a numeric value (e.g. 203) as a string in a WHERE clause might result in poor performance. Possible impacts of storing numbers as varchars: more space will be used, you won't be able to perform arithmetic operations, the data won't be self-validated, aggregation functions like SUM won't work, the output may sort incorrectly and more. If the column is numeric, remove the quotes from the constant value, to make sure a numeric comparison is done.
6. Use Numeric Column Types For Numeric Values (query line: 23): Referencing a numeric value (e.g. 204) as a string in a WHERE clause might result in poor performance. Possible impacts of storing numbers as varchars: more space will be used, you won't be able to perform arithmetic operations, the data won't be self-validated, aggregation functions like SUM won't work, the output may sort incorrectly and more. If the column is numeric, remove the quotes from the constant value, to make sure a numeric comparison is done.

Optimal indexes for this query:

ALTER TABLE `es_temp1` ADD INDEX `es_temp1_idx_id` (`Id`);

The optimized query:

SELECT
        WrappedData.* 
    FROM
        (SELECT
            ParentLeafNodeDataSource.Id,
            LatestDataSourceActivityLog.UserId,
            DataSource.Status AS StatusCode,
            (CASE 
                WHEN User.Name IS NULL THEN 'CompanyName' 
                ELSE User.Name END) AS `Username`,
Workflow.Name AS WorkflowName,
LatestDataSourceActivityLog.Timestamp 
FROM
DataSource,
Workflow,
es_temp1 AS ParentLeafNodeDataSource,
es_temp2 AS LatestDataSourceActivityLog 
LEFT JOIN
User 
    ON User.Id = LatestDataSourceActivityLog.UserId 
WHERE
ParentLeafNodeDataSource.Status = '203' 
OR ParentLeafNodeDataSource.Status = '204' 
AND Workflow.Id = ParentLeafNodeDataSource.WorkflowId 
AND LatestDataSourceActivityLog.DataSourceId = ParentLeafNodeDataSource.Id 
AND DataSource.Id = LatestDataSourceActivityLog.DataSourceId 
AND LatestDataSourceActivityLog.UserId = 1 
GROUP BY
ParentLeafNodeDataSource.Id 
ORDER BY
NULL) AS WrappedData 
ORDER BY
WrappedData.`Timestamp` DESC