The optimization process and recommendations:

1. Avoid OFFSET In LIMIT Clause (query line: 38): OFFSET clauses can be very slow when used with high offsets (e.g. with high page numbers when implementing paging). Instead, use the following \u003ca target\u003d"_blank" href\u003d"http://www.eversql.com/faster-pagination-in-mysql-why-order-by-with-limit-and-offset-is-slow/"\u003eseek method\u003c/a\u003e, which provides better and more stable response rates.
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. Replace In Subquery With Correlated Exists (modified query below): In many cases, an EXISTS subquery with a correlated condition will perform better than a non correlated IN subquery.

Optimal indexes for this query:

ALTER TABLE `tbl_activities` ADD INDEX `tbl_activities_idx_user_id` (`user_id`);
ALTER TABLE `tbl_subscriptions` ADD INDEX `tbl_subscriptions_idx_subscriber_id_subscribed_uid` (`subscriber_id`,`subscribed_uid`);
ALTER TABLE `tbl_users` ADD INDEX `tbl_users_idx_id` (`id`);

The optimized query:

SELECT
        tbl_users.id,
        tbl_users.user_name 
    FROM
        tbl_users 
    WHERE
        EXISTS (
            SELECT
                1 
            FROM
                (SELECT
                    act.user_id 
                FROM
                    tbl_activities act 
                INNER JOIN
                    tbl_users u 
                        ON act.user_id = u.id 
                WHERE
                    NOT EXISTS (
                        SELECT
                            1 
                        FROM
                            tbl_subscriptions s 
                        INNER JOIN
                            tbl_users u1 
                                ON s.subscribed_uid = u1.id 
                        WHERE
                            (
                                s.subscriber_id = 1
                            ) 
                            AND (
                                u.id = s.subscribed_uid
                            )
                    ) 
                GROUP BY
                    act.user_id 
                ORDER BY
                    COUNT(act.user_id) DESC LIMIT 0,
                    15) AS t 
                WHERE
                    (
                        tbl_users.id = t.user_id
                    ))