I'm not sure about the source of the lag, so I'm open to ideas on how to narrow it down. A comparison with the web interfaces shows about the same delay between operating a control and the on-screen response, so I'm leaning toward Telemachus or KSP itself.
Here's a snippet of the debug while quickly changing the throttle from max to off:
{"t.universalTime":17020.1764956399,"f.throttle":1}
Sending data: {"run":["f.setThrottle[1.000]"]}
{"t.universalTime":17020.29649564,"f.throttle":1}
Sending data: {"run":["f.setThrottle[0.367]"]}
{"t.universalTime":17020.29649564,"f.throttle":1}
Sending data: {"run":["f.setThrottle[0.015]"]}
{"f.setThrottle[1.000]":0,"f.setThrottle[0.367]":0,"t.universalTime":17020.41649564,"f.throttle":1}
Sending data: {"run":["f.setThrottle[0.000]"]}
{"f.setThrottle[0.015]":0,"t.universalTime":17020.53649564,"f.throttle":0.367}
Sending data: {"run":["f.setThrottle[0.000]"]}
{"f.setThrottle[0.015]":0,"t.universalTime":17020.53649564,"f.throttle":0.367}
Sending data: {"run":["f.setThrottle[0.000]"]}
{"f.setThrottle[0.000]":0,"t.universalTime":17020.65649564,"f.throttle":0.01499999}
{"f.setThrottle[0.000]":0,"t.universalTime":17020.65649564,"f.throttle":0.01499999}
{"f.setThrottle[0.000]":0,"t.universalTime":17020.65649564,"f.throttle":0.01499999}
{"f.setThrottle[0.000]":0,"t.universalTime":17020.65649564,"f.throttle":0.01499999}
{"f.setThrottle[0.000]":0,"t.universalTime":17020.65649564,"f.throttle":0.01499999}
Sending data: {"run":["f.setThrottle[0.000]"]}
{"f.setThrottle[0.000]":0,"t.universalTime":17020.77649564,"f.throttle":0}
Messages to Telemachus start with "Sending data:". You can see that Telemachus does not immediately respond with the correct value, but that it takes a bit, even though it acknowledges the change in its response.
On the other hand, the flag waving at the launch pad is not exactly smooth right now, so system load might be an issue...
To be honest, I did it just to see if I could. There are two other things I'd like to try now. I have one of those emergency stop buttons, and I think I can get a fully self-contained abort button. I'd also like to see a remote docking control.