I understand you want to know the impact transaction replication will have on the CPU utalization.
Kindly note that the CPUUtalization depends on the following factors:
- The amount of Load on the source database.
This is the amount of load of the On-Premises database that you are planning on moving to the AWS RDS (Target), the cpuutilization is dependent on the load, therefore if your load gets higher, so will your cpu.
- The number of transactional replication client that you create.
It is best practice to reduce load on the source database when exporting data from SQL Server databases.
After the migration has completed and you are still experiencing a very high irregular CPUUtilization, you can refer to RDS documentations for troubleshooting (Please take note of the engine types and refer to the documentation of the relevant engine).
I hope this helps.
 Importing and exporting SQL Server databases using native backup and restore: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/SQLServer.Procedural.Importing.html#SQLServer.Procedural.Importing.Native.Using.Restore
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