While the internal details of RDS instances is proprietary, this is my understanding after having conversations with AWS Support.

Automated Snapshots

These are the daily snapshots that are created automatically by your RDS instance. They have a "Snapshot Type" of "automated".

Performance is never impacted when automated snapshots are being created.

Manual Snapshots

When creating manual snapshots, creating the snapshot may affect the performance of other clients connected to the database.

Single AZ:

If your RDS instances are Single AZ, then creating a DB snapshot can result in a brief suspension of I/O that could last from a few seconds to a few minutes. The length of time depends on the class and size of your RDS instance.

Multi AZ:

If your RDS instances are Multi AZ, then creating a DB snapshot will not affect other clients. This is because the snapshot is created from the standby instance.

Exception: For Microsoft SQL Server, being Multi AZ does not prevent the I/O suspension.

Aurora Clusters:

Aurora clusters continuously and incrementally keep backup data. So they are constantly creating backup data. When you create a snapshot, it simply flags that point in time. So no performance impact or interruption occurs.

Additional References

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