Problem with Autosteer PGN and data rates

So Ive run into a problem with PGN exchange with me custom built Seed motor control board running on a mega where I loose connection with AOG. I know its because I am doing too much processing in the loop as I need to read the motor encoder a fan encoder among other things. I can see the watchdog timer creeps up quickly once i introduce the delay. I can reproduce the issue in the default machine_usb_v5.0 code by simply adding a delay(1) in the main loop.

Can anybody explain what is happening here? Is it simply that the serial buffer is getting overrun and so it never gets to receive a full message?

Thanks