Issues regarding performance on R series and C Series EC2 Instances

We are running an email processing application for one of our client, here they send the emails via smtp to our servers and before delivering the emails to the final MTA we are supposed to do some process on the incoming SMTP packet like adding tracker information. Currently we are doing this on an stand alone server at a data center in US which has Intel Xeon E3-1270v6 - 4c/8t - 3.8 GHz/4.2 GHz with SSD drives and 64 GB RAM.

Here we are able to process very easily 130 - 150 mails per second.

Now when we moved this to AWS EC2 on a c6a.4xlarge the same process is hardly processing 50 - 60 mails per second.

Is there any throttle on AWS or have we selected a wrong EC2 instance ?

Please guide.