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. 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.

Optimal indexes for this query:

ALTER TABLE `ImportReportHistory` ADD INDEX `importreporthistor_idx_requested` (`Requested`);
ALTER TABLE `ImportReportParameter` ADD INDEX `importreportparame_idx_parametername_importreporthis` (`ParameterName`,`ImportReportHistoryID`);

The optimized query:

SELECT
        TOP (@numberToRetrieve) ir.*,
        (SELECT
            CAST(iparm.ParameterValue AS datetime) 
        FROM
            ImportReportParameter iparm 
        WHERE
            iparm.ImportReportHistoryID = ir.ImportReportHistoryID 
            AND iparm.ParameterName = 'RangeStart') RangeStart,
        (SELECT
            CAST(iparm.ParameterValue AS datetime) 
        FROM
            ImportReportParameter iparm 
        WHERE
            iparm.ImportReportHistoryID = ir.ImportReportHistoryID 
            AND iparm.ParameterName = 'RangeEnd') RangeEnd 
    FROM
        ImportReportHistory ir 
    ORDER BY
        ir.Requested DESC