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.

Optimal indexes for this query:

ALTER TABLE `category1_to_category2` ADD INDEX `category1_category_idx_id_category1` (`id_category1`);
ALTER TABLE `category2` ADD INDEX `category2_idx_id` (`id`);
ALTER TABLE `category2_to_item` ADD INDEX `category2_item_idx_id_category2` (`id_category2`);
ALTER TABLE `item` ADD INDEX `item_idx_id` (`id`);

The optimized query:

SELECT
        c1.name AS c1name,
        c2.name AS c2name,
        i.name 
    FROM
        category1 c1 
    LEFT JOIN
        category1_to_category2 c1tc2 
            ON c1.id = c1tc2.id_category1 
    LEFT JOIN
        category2 c2 
            ON c1tc2.id_category2 = c2.id 
    LEFT JOIN
        category2_to_item c2ti 
            ON c2.id = c2ti.id_category2 
    LEFT JOIN
        item i 
            ON c2ti.id_item = i.id