The signal you detect at the Arduino pins is BEFORE the outgoing data gets into the USB system or AFTER the incoming data has left the USB system. Your 'scope is not viewing the USB system and your pictures do not invaildate what I said about the USB2 1 millisec delay.
Perhaps what you are really concerned with is how quickly, after it receives a incoming byte on the Rx pin can the Arduino present a byte on the outgoing Tx pin?
...R