This is because since decryption happens on the fly as data flows in, it cannot be faster than the pace of the incoming data. So it is expected and correct that decryption takes 8 times longer when the rate is configured for 125 kbps (S=8) compared to using 1 Mbps. For encryption, it is different, since all data is present from the beginning of the operation.
But, I am not using the on-the-fly decryption method. I am explicitly giving the Encrypted data as Input and waiting for the Decryption to complete and get the decrypted output. So, I would like to know if there is any configuration that can be used to get the decryption done in quickly like in the case of Encryption.
Ah, I finally get the question. No, that is not possible. The CCM module is not very flexible since it is specifically designed to work with the radio in BLE context. The decryption speed has to be one of the supported datarates.
