I am wondering if the following statements are right:
Both "adding read replica" and "upgrading DB instance class" improve read performance but result in difference cost and maintenance.
"upgrading DB instance class" improves also write performance.
Under https://calculator.aws/#/createCalculator/RDSMySQL there is a tip
You can add up to 5 read replicas to each database instance for enhanced read performance and increased availability. Read replicas can be Single-AZ or Multi-AZ, and can have the same or a different instance size as the database instance.
Add each read replica to the AWS Pricing Calculator as an additional Amazon RDS for MySQL service.
While according to https://aws.amazon.com/rds/mysql/pricing/
the prices of the next higher classes cost double the previous lower one.
It seems to me that upgrading to a higher class wins over adding a read replica (same price, additional write improvement) but I have no real experience with the matter so far.
Any hints, comments?