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 `user_tasks` ADD INDEX `user_tasks_idx_user_id_status_created_at` (`user_id`,`status`,`created_at`);
ALTER TABLE `user_tasks` ADD INDEX `user_tasks_idx_created_at` (`created_at`);
ALTER TABLE `user_tasks` ADD INDEX `user_tasks_idx_user_id_status_created_at_2` (`user_id`,`status`,`created_at`);

The optimized query:

SELECT
        TIMEDIFF((SELECT
            user_tasks.created_at 
        FROM
            user_tasks 
        WHERE
            user_tasks.user_id = 1 
            AND user_tasks.`status` = 'pending' 
        ORDER BY
            user_tasks.created_at DESC LIMIT 1),
        (SELECT
            user_tasks.created_at 
        FROM
            user_tasks 
        WHERE
            user_tasks.user_id = 1 
            AND user_tasks.`status` = 'pending' 
        ORDER BY
            user_tasks.created_at ASC LIMIT 1)) AS pending_time