Problem is that I need to manage a complex device, with minimum 2 servo motors and few sensors. So I can't just send 2 bytes. I need to send one byte as identifier for 2 other bytes. In other words, they must go in command-parameter formation. Now, as far as I currently see, bytes in formation: "sXX" must have terminating byte, so it is maybe more like "sXX\n" which is already 4 bytes. It seems that lengthy commands like "servox 2179\n" are drag.
I really didn't expect that bottleneck is going to be on Processing side, since it runs on PC with all that hardware power attached. I tried some other Processing code, that is sending only a single byte and that runs perfectly smooth, even at 9,600 baud.