The optimization process and recommendations:

1. 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.
2. Prefer Inner Join Over Left Join (modified query below): We identified that one or more left joined entities (e.g. `Groups`) are used in the 'where' clause, in a way that allows to replace it with an optimized inner join. Inner joins can be fully optimized by the database, while Left joins apply limitations on the database's optimizer.
3. Prefer Inner Join Over Left Join (modified query below): We identified that one or more left joined entities (e.g. `GroupValue`) are used in the 'where' clause, in a way that allows to replace it with an optimized inner join. Inner joins can be fully optimized by the database, while Left joins apply limitations on the database's optimizer.
4. Use Numeric Column Types For Numeric Values (query line: 16): Referencing a numeric value (e.g. 25) 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 `ClientGroup` ADD INDEX `clientgroup_idx_groupcode` (`GroupCode`);
ALTER TABLE `Clients` ADD INDEX `clients_idx_clientkey` (`ClientKey`);
ALTER TABLE `GroupValue` ADD INDEX `groupvalue_idx_groupvalue_groupcode` (`GroupValue`,`GroupCode`);
ALTER TABLE `Groups` ADD INDEX `groups_idx_groupcodekey` (`GroupCodeKey`);

The optimized query:

SELECT
        cli.ClientNo AS clientno,
        cli.ClientName AS clientname 
    FROM
        Clients AS cli 
    LEFT JOIN
        ClientGroup AS cg 
            ON cg.ClientKey = cli.ClientKey 
    INNER JOIN
        Groups AS gc 
            ON gc.GroupCode = cg.GroupCode 
    INNER JOIN
        GroupValue AS gv 
            ON gv.GroupCode = gc.GroupCode 
    WHERE
        gc.GroupCodeKey = '25' 
        AND gv.GroupValue = 'EXCLUDE'