It's a best practice to create a read replica when you don't expect a heavy database load. This is because the read replica creation process might cause I/O latency until it's completed. To reduce I/O latency during read replica creation and on your source RDS DB instance, you can use a High availability (Multi-AZ) for Amazon RDS.
Note: To improve performance, configure your read replica so that it's identical to its source database. If you use the Amazon RDS console to create a read replica, the source database settings are used by default.