Significantly Slower DB Dumps on AWS RDS After Turning on Encryption?
mysqldump reads all of your data.  If the encryption is at the disk, then it takes time to decrypt the data before mysqldump can access it.  Hence, the total action is slower.

Do you have any metrics on how much slower (percentage)?

Also, since RDS does key management, there may be a slight, one-time, delay to get the key.