You’re welcome; I gathered the info looking a bit further in the Arduino docs, everything was already there - if you only know where to look, as usual
I followed the same path, and used a serial device I already had.
I have an original Arduino 2009 and a Rainbowduino v2 - my results are the same as yours, and from the various posts on the forum you see that quite a lot of people have had the same problem in the past.
The only thing I can think of is reflashing one of the two bootloaders and use a different version (maybe the older one, which was slower). But you would need an AVR programmer of some sort for that, AFAIK.
Antonio