EX4100 file transfer with encrypted volume / remove encryption

I have the EX4100 configured an raid 5 array of 4 10 TB drives. I also have the PR4100 configured in the same way. The only difference is that the EX4100 has encryption applied to the RAID 5 volume.
I am getting transfer speeds to the PR4100 that saturate gigabit lan (115 mb/s), but on the EX4100 I’m only getting about 65 to 70 mb/s.
Question I have is whether the difference is hardware related, or due to the encryption?
If it is the latter, is there a way to remove the encryption without losing data? I have the password.

Thanks for the quick reply. No obvious option to remove encryption in the web interface.
Clearly a school boy error in enabling it!
The PR4100 provides a full back up - it is just a huge amount of data to transfer, and would rather not!