The optimization process and recommendations:

1. Avoid Subqueries (query line: 15): 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'.

Optimal indexes for this query:

ALTER TABLE `tb_DeviceLog` ADD INDEX `tb_devicelog_idx_inventorylogid_trackedon` (`inventoryLogId`,`trackedOn`);
ALTER TABLE `tb_InventoryLog` ADD INDEX `tb_inventorylog_idx_assignedtocustid` (`assignedToCustId`);
ALTER TABLE `tb_Logins` ADD INDEX `tb_logins_idx_username` (`userName`);

The optimized query:

SELECT
        DL.lat,
        DL.lng,
        DL.speed,
        DL.trackedOn,
        IL.speedLimit,
        IL.deviceName,
        IL.deviceId,
        isnull(DeviceIcon,
        'Content/images/beach-car.png') AS DeviceIcon 
    FROM
        tb_DeviceLog DL 
    INNER JOIN
        (
            SELECT
                tb_DeviceLog.inventoryLogId,
                max(tb_DeviceLog.trackedOn) AS MaxDate 
            FROM
                tb_DeviceLog 
            GROUP BY
                tb_DeviceLog.inventoryLogId 
            ORDER BY
                NULL
        ) IDL 
            ON DL.inventoryLogId = IDL.inventoryLogId 
            AND Dl.trackedOn = IDL.MaxDate 
    INNER JOIN
        tb_InventoryLog IL 
            ON DL.inventorylogid = IL.id 
    INNER JOIN
        tb_Logins LGN 
            ON LGN.customers_id = IL.assignedToCustId 
    WHERE
        LGN.userName = 'cadmin'