So, RDS is generally always the biggest cost in anyone's billing, as far as I have known.

Because at end of the day all the scaling infra comes down to database.

As we were able to optimise our queries and re-architect some of the API logic and design, we were able to reduce our RDS from db.r5.4xlarge to db.r5.large for 3 of our large databases.

So you see, it almost reduced our billing of just one service by a factor of 4.