Serial Rate drops after arming


I am facing an issue that I wanted to check if anyone else has noticed using Pixhawk2.1 and Ardupilot 3.5.3. As soon as, I arm the serial rates drop significantly, and as soon as I disarm they recover. Do you have any ideas what could be the cause of that?

I have been using the same rates with 3DR pixhawk for a while 20Hz for everything, and I havent noticed it before.