The optimization process and recommendations:

1. Avoid Subqueries (query line: 19): 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 `user_info` ADD INDEX `user_info_idx_user_id_email` (`user_id`,`email`);

The optimized query:

SELECT
        user_info.email,
        EXTRACT(year 
    FROM
        timestamp_seconds(time_stamp)) AS Year,
        EXTRACT(month 
    FROM
        timestamp_seconds(time_stamp)) AS Month,
        EXTRACT(day 
    FROM
        timestamp_seconds(time_stamp)) AS Day 
    FROM
        Mixpanel.events_log 
    JOIN
        Mixpanel.user_info 
            ON user_info.user_id = events_log.user_id 
    WHERE
        time_stamp IN (
            SELECT
                min(time_stamp) AS time_stamp 
            FROM
                Mixpanel.events_log 
            INNER JOIN
                Mixpanel.user_info 
                    ON events_log.user_id = user_info.user_id 
            WHERE
                event = 'change_screen' 
                AND screen = 'canvas' 
            GROUP BY
                user_info.email 
            ORDER BY
                NULL
        )