Are read replicas heavy reading in affect masterDB?
From your tags, I guess its for AWS RDS Aurora. Generally, Aurora's design differs from native databases. It has a distributed storage system. 

Of course, if you huge select will affect the master when you have MyISAM in place. But Aurora completely designed with InnoDB. So it won't. 

Not sure whether your replica and master are in the same AZ. Its a good practice to use different AZ in Aurora. 

Due to its PIOPS, your massive select load will not affect your master anyway. If you have a predictable load on a particular time, use Aurora autoscaling to share the loads between multiple read replica.