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. Use UNION ALL instead of UNION (query line: 17): Always use UNION ALL unless you need to eliminate duplicate records. By using UNION ALL, you'll avoid the expensive distinct operation the database applies when using a UNION clause.

Optimal indexes for this query:

ALTER TABLE `people` ADD INDEX `people_idx_personpk` (`personPK`);

The optimized query:

SELECT
        1 
    FROM
        people 
    WHERE
        people.personPK NOT IN (
            SELECT
                attendees.personFK 
            FROM
                attendees 
            UNION
            SELECT
                photo_tags.personFK 
            FROM
                photo_tags 
            UNION
            SELECT
                email_subscriptions.personFK 
            FROM
                email_subscriptions
        )