Do you think that using a 2600-daptor helped to reduce the lag a little, and that there could be a difference in lag between the 2600-daptor and the default RS323 ports on the R77?
In my testing, I don't think there was any difference between using the Stelladaptor on the laptop and the joystick ports on the RetroN 77. At least none that I could discern.
When I experienced any lag, I experienced it equally on both systems, and it was always in line with amount of lag seen using the HDMI lag tester.
I only used the Stelladaptor on the laptop. For the RetroN 77, the joysticks were always plugged in directly. In either case, the responsiveness felt identical between them across all tests. (I should note that I only tested joysticks - not paddles. I don't have the necessary cables to test a Stelladaptor on the RetroN 77.)