Indeed, I didn't mention that I tried to increase the baud rate up to 256000. Exactly the
same behavior (8ms ok, 7,6,5ms packets lost) and percentage of data packets lost (2.3%).
Data is lost in packets, not in single bytes.
27 bytes * 10SerialBits/byte / 0.005 s = 54000 baud... less than half the communication speed I'm using.