Serial data transmission is relatively slow, particularly at 9600 baud. Since the two Arduinos are directly connected, they can communicate MUCH faster than that. Try increasing the baud rate, to the maximum supported speed, 115200. That's 12 times as fast, so will take 1/12th as long.
Also, one of your Arduinos is waiting half as long as the other. The faster one will eventually overflow the serial buffer on the slower one.