I have a SQL query when joining several large tables. My database consists of millions of rows across the users, orders and transactions table. Here is the query

    SELECT u.name, COUNT(o.id) AS order_count
    FROM users u
    JOIN orders o ON u.id = o.user_id
    JOIN transactions t ON o.id = t.order_id
    WHERE t.status = 'completed'
    GROUP BY u.name
    HAVING order_count > 10;

I ensured that there are indexes on all the necessary fields. users.id, orders.user_id and transactions.order_id. However the query becomes significantly slower as the dataset size increases.

The order table is approximately 5 million rows.
The transactions table is approximately 10 million rows.
The database is running on MySQL 8.

What optimization techniques or query restructuring could help improve performance in this case?