If your database queries are so bad that you can get such big improvements by changing the queries and indexes, then obviously focus on that, not the client pro

If your database queries are so bad that you can get such big improvements by changing the queries and indexes, then obviously focus on that, not the client programming language. But once you've done that your programming language is responsible for a larger ratio of the total time, since it runs in series to the database query, not in parallel. The benefit, in my experience, is much larger than 10% then.